Depends what you want a new card for - ATI and NVIDIA have their respective strengths in terms of rendering capabilities for gaming, and some game engines are optimised to work best with specific chipsets.
The X850 is getting a bit long in the tooth now, so should be cheap (ATI's latest is the X1900).But I've got the mobility X800 in my laptop and it can max-out AA settings for example in Doom 3 and HL2 and give good framerates.
Check out the techie review sites for comparison between cards you're interested in. Many out there but
Hexus and
Toms Hardware are well respected