I flashed the new SPL and radio that also came with the T-Mobile Froyo OTA giving myself an extra 15 MB of RAM. This post attempts to compile the background and links to the knowledge needed to do this safely. Because this is much more complicated than rooting with universal androot, and you can very easily brick your phone, this should not be done if you don't fully understand the process and aren't comfortable using the command line. While I believe someone may have created flashable signed zips for doing this, the recommended way is to use fastboot.
ROM Memory Swap Total (KB) Total (MB)
Rooted old SPL/radio
Rooted new SPL/radio
Without swap, that is an additional 14.8 MB of real memory.
First, a few things you need to know before proceeding:
Perfect Secondary Program Loader
The ht-03a shipped with a "perfect" SPL that does not allow remote fastboot. Therefore, an interim engineering SPL will have to be flashed in order to use fastboot to flash the new SPL (1.33.0013) and radio. The new SPL is also perfect, but a version was modified for the Dream/G1 (1.33.0013d) that allows a few fastboot commands, such as fasboot erase system -w for a complete wipe. Fortunately, 1.33.0013d has been confirmed to also work on the ht-03a, a Google-branded 32B magic.
Even with the modified "d" version of the new SPL, you cannot remote boot a recovery image, flash new splash screens, flash a new recovery image (if for example yours became corrupted). You will have to flash an engineering SPL, do what you must, then reflash 1.33.0013d. Unlike flashing ROMs, flashing SPLs is not 100% risk free.
Amon_RA Recovery Recommended
I'm sure this will probably get some feathers ruffled, but I am leery of clockwork recovery and ROM Manager. As outlined here, this is not the developer's fault but a low memory issue. Perhaps this new SPL and radio will fix that, but the fact remains that there have been a number of instances Dream and Magic users getting stuck with no working recovery. With an Engineering SPL, this is no problem because fastboot flash recovery recovery-RA-sapphire-v1.7.0G.img fixes this, but with a perfect SPL, it will be much more difficult, as in making a goldcard difficult.
Another fact is that I have yet to hear of this ever happening with Amon_RA's recovery, and ezterry at XDA is suggesting to use Amon_RA. However, using Amon_RA means you can't use ROM Manager.
(NOTE: ht-03a users need "G" versions of Amon_RA)
Patched Kernel Required
If you are like me, you never cared about kernels. Yeah, you might have been interested in testing out some of the overclocked kernels but you were perfectly happy to use whatever the developers included with the ROM, which will likely be the most stable way to go but maybe not the fastest. Stability versus bleeding edge. For the time being, those days will be over. All of the cooked ROMs available right now will need a patched kernel that may or may not be included with the ROM.
I have no idea what is the "best" kernel. It will depend on what makes it the best for you.
Start with ezterry's XDA write up with links to the radio and SPL. If this doesn't make sense, stop and read more.
If you don't already have one, read the instructions on flashing an engineering SPL.
The SPL in the above link comes from this list of engineering SPLs.
Finally, you'll want to understand how to flash Amon_RA's recovery because it is less prone to problems than clockwork.
If you plan to use CM 6.1, ezterry's CM Forum post contains a port with the new radio.
Engineering SPL (I am using 1.33.2005. Many others are using 1.33.2009. IIRC, there might be issues with 1.33.2010 and CM, so 2009 is probably best.)
fastboot utility installed on your PC
Android SDK on your PC to allow you to use ADB (Perhaps this is technically not required but at this point of getting into your phone you ought to have it installed.)
What I did
The T-Mobile US Froyo OTA with a cyanogen kernel I've been running lately is by far my favorite ROM due to speed and stability. I've mentioned missing some CyanogenMod features, but when it comes to my phone, stability trumps the bleeding edge. So I 1) did a nandroid backup of my current ROM, 2) updated the SPL and radio, 3) restored the backup, 4) flashed the patched kernel, and 5) picked up where I left off with no loss of data on my phone. The actual work involved for me was about 10 minutes because I already had an engineering SPL and Amon_RA. I spent infinitely more time writing about this than I did actually experiencing it.
So far, all seems fine, but I'll need a bit of time to test. Wifi and GPS are working, and the phone does seem snappier. I haven't had a chance to see how battery life is. There are two choices of radio, 2.22.27.08 or 18.104.22.168. The former came with the OTA. The later is a newer radio. Depends on who you ask and where they live, which is better/faster/easier on the battery. For now I am using 2.22.27.08.
Rebooted to recovery and nandroid backup my current ROM.
Followed ezterry's instructions on XDA
Downloaded all necessary files and checked the MD5 sums against what was published.
dream_2708kit.zip which includes 2.22.27.08 radio and 1.33.0013d SPL
Kernel installer for CM Nightly 271
Mounted my SD card, copied the kernel installer to the root of my SD Card, and rechecked the md5 sum.
fastboot flash radio radio-2.22.27.08.img
fastboot flash hboot hboot-1.33.0013d.img
fastboot erase system -w
fastboot erase boot
Reboot to recovery and restore ROM.
Pray (actually I used ddms watch all the boot messages)
New v Old SPL/Radio
HBOOT Radio Context Type Security SPL Maker Fastboot USB
*fastboot erase allowed
Here are the hboot/radio combinations I've had on my ht-03a, beginning with the Docomo stock configuration. After that I used the same radio with an engineering SPL. This was prior to universal androot with no one-click option for 1.6 Donut; An engineering SPL was required to root. Finally, I'm back to a mostly perfect SPL but with an additional 15 MB of real memory.
What most of you will need to do
NOT follow these steps but use them as A Docomo ht-03a SPECIFIC REFERENCE for ezterry's instructions.
If you are unrooted, root your phone and flash a custom recovery. clockwork is fine to start with.
Download and flash an engineering SPL from recovery (e.g., 1.33.2009, though I have been using 1.33.2005). VERIFY MD5 SUM after download and after copying to SD card.
Reboot to fastboot (power + back).
Download Amon_RA recovery (NOTE: that the code in exterry's guide is specific to a dream ht-03a users need a sapphire recovery tagged "G". "cyan" refers to the color scheme, not cyanogen.) VERIFY MD5 SUM!
Using the terminal, cd to the directory with the img file and fastboot flash recovery recovery-RA-sapphire-v1.7.0G.img which assumes you have added fastboot to your path (Instructions for mac/linux here. Windows users on their own.)
Continue as I did.
Restore your previous ROM and flash the "2708" ported kernel of you choice.
Or follow these directions for CM 6.1 after rebooting to recovery.