The are really close performers and would trade blows depending on the game/settings. The X850 pro should OC better, but was really a strange product for ATI to release as originally it was priced higher than the XL and performed on par. It did offer VIVO, which the original XL's didn't have? Hope that helps.
Does anyone know of any benchmark sites to compare the performance of these cards?
Again any info is greatly appreciated,
You should be able to find plenty of reviews on the web. If you are searching for specifically AIW x800xl vs x850 Pro comparisons you may come up short. The plain x800xl is the same as the AIW card performance wise so just search that.
These cards can get a bit complicated depending on which variant you are looking at. Some have more pipelines that can be unlocked depending on core like the 850Pro but only the Vivo one will do it as the others were lazer cut, and so it goes on. If you have two specific cards you are looking at getting let us know what they are AGP/PCIE is relevant also and we will see what help we can be.