At that resolution, I'd recommend one of the following: gtx460, HD6870 or HD6850.
The 6950 may be more than you need at that resolution, unless you find you must have exceptional FPS at all times in games like Metro 2033/Crysis with everything maxed out. You should have plenty with the other 3 I mentioned.
Well i am the type that likes to play FPS and all games at 60+FPS with maxed out settings and i was somewhat dissapointed when i bought my 6870 and it couldn't max out BFBC2 and give me 60+FPS.
If i was you and you were me i would get a 6950.It is futureproof and for $50 more you can't really beat that compared to the 6870.Also whenever or if you do plan on getting a 1080p monitor the extra 2G the 6950 has will help you out and futthermore an eyefinity setup.
Here is a review on the 6950 and some power consumption charts you might want to take a look at.I'm guess the chat is display the Total Power Consumption meaning everything together is a lil less than 500watts so you would still be ok with your current PSU.
1600x900 so i was even more disapointed that i couldn't get 60+FPS
Have a Phenom ii x4 @3.8ghz and 4gdrr3 so the 6870 is being pushed to it's limits but still can't get the performance i hope for.With that said i really think you should get the 6950 but that just my opinoin.
Based on your system specs (and predominantly your resolution of 1600x900) I'd go with an ATI 5770 (or ATI 5850 just cause they are about the same price right now). For nVidia's side of things a GTX 460 (even 768MB) would be plenty of power. Anything more than the ATI 5770 (even the GTX 460 or ATI 5850) is more than you need for most games.
Now, if as others have brought up, you want to max out Crysis or Metro 2033 (which are absolute beasts) then you could consider something more impressive (GTX 470, ATI 6870, etc). But that's just two games that don't perform well, but look nice.