There have been a few threads on this, and despite the occasional fanboyism (or accusations there of) I find them kind of enjoyable. I guess that's because I'm still using my x1800xt and I've been holding off on making a commitment to either manufacturer. But I have to decide some time soon.
Are you considering running 2 graphics cards? Then crossfire is probably your best choice which puts you in the ATi camp. From what I've heard, SLI is only worth considering if you're building an i7/x58 rig.
Another option is a dual GPU card. The most affordable one is the ATi "X2" type. Last I checked, a 2Gig 4850X2 was a pretty good deal. The "best of the best of the best" is the nVidia GTX 295 (it also costs the most of the most of the most).
SLI/crossfire and dual GPU cards are not without their problems (not all games support SLI/crossfire and the drivers may have some issues). You generally only have to resort to these types of solutions if you're monitor is high resolution (say over 1680x1050), but in some cases you might see a gain at lower resolutions.
That brings us to the single card/single GPU category. In terms of absolute performance, the GTX 285 rules the roost at $310. But, it is only perhaps 5% (at most) better than the HD-4890 at $220 (you can get an MSI one for just under $200). The GTX 275 is about the same as a 4890 (better on some games, worse on others), and it costs about $250. In one benchmark, the 4890 was generally better than the GTX 275 at resolutions 1680x1250 or less.
So right now it's hard to argue against the 4890 in terms of absolute price and performance. The other distinguishing characteristics between the current nVidia and ATi cards seem to be a draw (for example nVidia has PhysX while ATi has dx10.1). If you care about the performance of particular game then you should be a bit more careful and check the benchmarks. Some games are effected more by your ram/cpu than by the graphics card!
So that raises the question, why would you pick an nVidia card? Well, if you have an extra $100 burning a hole in your pocket and you want the absolute best single GPU card, then the GTX 285 might make sense. The GTX 285 has also been out for a good long while, so the drivers are a bit more stable (the GTX 275 and HD 4890 are pretty new). I'm not sure about sound levels, but I thought the GTX 285 might be quieter. I'm really not sure about that.
EDIT I should also add: aren't we lucky to have ATi and nVidia? They both try to distinguish themselves with different and compelling technologies, and they also have to keep the main stream happy. Unless a game manufacturer is getting subsidized by ATi or nVidia, they will try to make their game work well on both. Capitalism meets the two party system.