As promised, some benches of my OC'd X800XL.

First the hardware setup:
Mem - 205MHz DDR @ 2-2-2-5 1T Trc = 10 + Trfc = 14
CPU - 2660MHz A64 Venice @ 313HTTx8.5
Mobo - MSI K8N Plat. (Neo4 Ultra)
CPU + GPU H2O'd (homemade yummy goodness).

The voltages have been cranked up just a bit.
Vcore = 1.73v, Vddq = 2.02v and Vdd = 2.00v

Core/mem 500MHz / 600MHz (yeah, not bad eh?)
3DMark 2000 = 26253 marks
3DMark 2001 = 26626 marks
3DMark 2003 = 12490 marks
3DMark 2005 = 5912 marks

The core@510 freezes after a few secs, 505 might work but I like to keep some buffer.
The mem@600 is artifact free, but only at core=500MHZ. At 400MHz core the mem@600 does give some artifacts... dunno why.

I kick the X800XT's ass just looking at the stock clocks, but comparing scores in 05 I even slap the X800XT PE's behind! (if the chart I looked at was correct).
Mind you, overclock those other cards and I'm right back on the end again.

Stock it did 440/580 and I didn't check if 580 was the ceiling.
Now it's +60/+20 compared to non modded and +100/+110 on stock.

All in all, not bad

Edit:
At 505/605 (roughly a 1% increase in speeds) I get only 10 marks extra.
10 on top of 5912 is 1.7% extra performance... where did the rest go? I'm guessing a bottleneck somewhere else than the videocard.
Perhaps a 2660MHz Venice just isn't powerfull enough...