I had one of these for 8 months, it's a great card. (until I got my 6800 and 6800GT, the best I had ever had)
I wouldn't flash the bios for a few more frames on this card.
a. The performance difference is trivial, you wouldn't be able to tell in use
b. having an XT bios doesn't give you the extra 128MB of better RAM
c. see above, and wonder why anyone would void there warranty for that?
OCing/flashing, etc is only worth it when you can get some noticeable gains. (e.g. my 5800NU 400/800 ran solid at 500/1000- that is a noticeable difference)