One thing to think about is if you care about PhysX in it's current incarnation. If not, then the field is open and I would personally get the 4890 since most reviews I have read put it ahead in the "low" res of 1680x1050. If you do care about PhysX and want to play games like Mirrors Edge with the extras then the GTX 275 is only real choice in your price bracket.
As a user of both nVidia and Ati cards (my 4870 is my at home card, my 8800GTS 512 is my LAN card) I don't find any noticable difference in drivers between them. I like the ATI control panel slightly better but find the nVidia drivers always seem more polished though I'm not sure why. In actual performance of new games (Empire TW, Stalker CS) I am primarily using my 4870 since thats in my home pc and I have noticed no issues besides the occasional texture problem. My lanbox is used for playing mostly older engine based games like CS:S, UT3, TF 2, WiC, and L4D. I have noticed random texture issues with most of those games but again, not really an issue.
All that is to basically say don't worry about the drivers. Ask yourself if the somewhat limited, but kinda cool, PhysX is worth if. If not, then look at game performance in the games you play. Really it comes down to the deals, right now I have found more 4890's on sale for cheaper than the GTX 275's but that can change at any moment. If you do go ATi I would get an XFX for the warranty, if you go nVidia and eVGA, BFG, and XFX all work great as well and have good warranties.
Good luck!
EDIT* As a note to your original question I did play Crysis Warhead at mostly very high settings using my 4870 512 at 1680x1050. I don't recall having any issues with FPS though it has been quite some time since I played it. I played the first Crysis on both my old 7800GT and 8800GTS 512 and neither could give me the smooth play I wanted though the 8800GTS came very close. Of course these are older cards so they should not be considered.