Yeesh - sorry haven't been around for a bit.
But I see a bunch of Flashing How-To Questions.
- If your new to Flashing... READ THE STICKIES! There are even youtube videos showing you exactly how to do this! (You can skip to the good parts btw)
- Make sure you understand how to RECOVER YOUR PHONE. I've seen too many people on the forums post the inevitable OMG MY PHONE IS BRICKED because they failed to understand how to restore their phone before they started flashing willy nilly.
- If you've just started flashing - I would suggest trying the AIO Toolbox - it has everything you'll need to flash a ROM.
- And lately this seems to becoming more prevalent so this is just my opinion on the matter... Please folks don't just go to trying to install a beta Cyanogen 7 ROM that someone just got up and running if your new to flashing.
- I would highly recommend flashing a well known ROM that has clear-cut directions. To prevent a flame war... I can't recommend which one - but it will be fairly obvious what they are

And finally.. if you've already flashed a ROM and you wish to flash a new one:
You CAN flash "over the top" of the existing ROM and 99% of the time you shouldn't have any issues.
BUT... the recommended way to safely flash a new ROM: (I've done this no kidding at least 75-100 times on my phone so the nay sayers can go nay all the live long day

)
Restore your phone back to stock using ODIN 1-Click
You can go the extra step to format your internal SD Card but this is NOT necessary.
Then follow the direction of the chef/dev for flashing the new ROM - typically it is:
- Root your device after restoring with ODIN 1-Click
- Install ROM Manager
- Copy the ROM to your SD Card (internal NOT External)
- Boot into Recovery using ROM Manager
- Choose Install from *.zip
For me - I cheat but still Odin 1-Click to restore the device. This has prevented issues on potential bad flashes or having issues when I change ROMs - And I can restore my phone in 15 minutes or less.
I have a ROM Manager "update.zip" I keep on my phone. It's small and allows me to go into advanced recovery without rooting my phone.
- I use Odin 1-Click to go back to stock.
- I install the new ROM onto the SD Card
- I usually go into the SD Card and remove any files/folders etc.. that I know are not normally there. (this step isn't needed its just what I do) Make sure you understand your file system before deleting stuff though please.
- I use ADB to reboot the phone into recovery (I have a build 1008 phone and the buttons NEVER work when I restore it to stock).
- Flash the new ROM
- Reboot ..
- Setup the phone initially (Sign into Google and wait for it to sync which is usually the longest step of flashing a new ROM)
- Download Titanium Back
- Restore from Backup I have on my SD Card (yup.. internal SD - though I keep a copy on my External as well - Odin doesn't wipe your SD Card - just re-flashes your phone back to stock.)
- And I'm done - in less than 15 minutes typically.