The X800XL is a terrible overclocker, that has been proven. The X800 Pro is actually better at overclocking and can wipe out the X800XL when overclocked.
All that aside, don't get an ATI card unless you plan on upgrading your card again in 6 months, or just don't care if you are stuck playing games in DX8.1. Dale is right in that games still look good in DX8.1 mode, but they look even better in DX9-SM3, which currently only Nvidia GeForce 6 series cards can do.
Question is, why get an ATI card without SM3 when you can get one with SM3 that performs just as good in all games for the same price, that is also an overclocker! I say get 6800 GT, you won't regret it, like you will if you get that ATI card.