Have you tried flashing a stock 2.1 firmware first, say I5800XXJK1 ? I know I had problems the first time I tried upgrading to 2.2. However I follow the steps in the guide exactly every time, particularly the hard resets before and after flashing etc, and don't have any problems.
Somebody is asking exactly the same question on xda forums
[Q] 5800 fails to Update during flash - I.e. Bricked - xda-developers