LOOK at what kies offers you before flashing if it's not JPM/JP6 then its not the froyo rom described in this thread.
Since some of the threads are buried and out of date I figured I would post this once more. Please read the whole post before flashing it can save you from a lot of hassles later.
Make sure your phone and network uses the same bands as this rom. In other words don't flash this on a US device that needs the 850 band since this rom is for europe.
****If your phone does not have a working 3 button recovery mode or if you don't know what that is then visit this link for more information before flashing your device****
I don't know if this is safe on branded, carrier locked devices.
This should be obvious but backup anything you don't want to lose.
First uninstall all lagfixes
Start up kies and connect your phone. Wait till kies recognizes the sgs.
Go to firmware upgrade.
If it doesn't report a JPM/JP6 upgrade close the upgrade window but do not close kies!
Go to windows start > run > and type "regedit"
Navigate to HKEY_CURRENT_USER/Software/Samsung/Kies/DeviceDB/
You will probably see a bunch of numbered keys (they look like folders) Find the key/folder that has strings that match your current firmware. So if you are on JPK look for that in the keys. If you have more than one galaxy S (it can happen) then match the imei with the phone you want to upgrade.
Change the following values so they look like this:
No go back to kies and click on firmware upgrade again. You should now see the JPM upgrade.
First boot can take a while so let it sit for a couple minutes while you stare at that glowing S
Doing this will not change your product code.
3 button recovery mode still works.
This won't wipe your phone's apps, settings, messages or SD card. (at least not for me coming from jpk)
It doesn't matter what firmware you are on we are just using the JF3 value to fool kies.
You will lose root if you were rooted. New root by Chainfire can be found here.
If kies crashed while extracting the firmware then try an older version of kies. If kies sees the update but still refuses to flash then try running kies as an administrator.
If your phone really hangs for more than 5 minutes at first boot then something probably went wrong. In that case start up in 3 button recovery mode and do a factory reset. Dont blame the registry hack, its kies that's the problem. As always flash at your own risk
If your android market is missing after flashing read the following thread for a solution: http://forum.xda-developers.com/showthread.php?t=810230
If the phone flashed ok but the rom doesn't run properly, force closes etc, then you can try entering 3 button recovery mode and doing a clear cache and a factory reset. Doing this will wipe your phone.
If you have lost your data connection after flashing you may have to re-enter your APN settings. Your can find this information on your carrier's website.