From CustomPc's benchmarks, the X800XL outperforms the 6800GT in Farcry and, in lower and mid-range resolutions, Half-Life 2. The difference at higher resolutions in HL2 though seems to be in the 6800GT's favour, although only roughly 5fps.
I haven't seen benchmarks of an X800XL running on an apg slot, but I'm interested to see if the architecture difference will make it perform better or worse. The magazine recommends the X800XL due to its low price, just over £200 for the card, which will allow you to play graphic-intensive games at a high fps. The HSF is also a lot quieter, compared to the behemoth that is the standard 6800GT HSF.
On the flipside, the Nvidia has SLI technology readily available, and there's also Shader Model 3 support, which might be useful for upcoming games (if they decide to support it!).