It really depends how much you game, and how much you're willing to spend. If you're an average gamer, than a mid range card is for you. The Radeon 9600XT is a great card for a good price, although you may want a 9800 Pro since they're realtively cheap and are moving into that "mid range" bracket, to be honest. As for nVidia mid range, the FX5700 is a good card as well for the same price.
As for high end, the X800 just got released, and you can grab one on eBuyer for around £290. They're very, very good, and will cope with the new games such as Half Life 2 and Doom 3 with ultimate ease. It's AGP 8x compatable etc etc, all the usual blurb really. As for nVidia's new offering, the 6800, it's also a damn good card, and I heard that it was only the reference card given to reviewers that would take up two slots. I hear that the official release card will only take up one slot (AGP), but it'll still require a heck of a lot of power. So to be honest, if you're looking for that sort of power for that price, I'd got for the ATI X800.
As for driver issues etc, ATI have gotten a lot better with their drivers recently, although nVidia do have excellent drivers, very easy to set up. When I was building my friends computer with an ATI card, I was a bit unsure when the drivers basically did three setups (I forget what for now, one for actual drivers, two others for Windows stuff I think). nVidia just does it like that.
Oh, and as for PCI Express, that won't be avaliable in motherboard or graphics form until much later in the year. Intel might shift something out for summer, though. For now, both cards are AGP 8x.
Both companies have great cards, though I'd say ATI still have that slight edge over nVidia at the moment.