Why won't Kies let me tried to update to Froyo again after a failed upgrade attempt?
Make sure you have the official drivers installed from Samsung.com, then reboot your device and PC and try again.  You can also try putting it into download mode before using Kies:


Remove the battery
Hold VolumeDown plus the Home key plus Power.  While holding,

Reinsert the battery.
Release the power button, still holding the others, about 1 second after the white "GT-I9000" text appears.
After a few seconds a yellow triangle with an android in it should appear, this is Download Mode.  If it doesn't, try this again while holding Power the whole time.