well if power isn't an issue and you are only allowed 1 slot, then you should get the best that you can afford.
The best cards handling 1650 x 1080 are:
GTX 280 FTW 1GB edition
GTX 260 OC (with 216 cores)
Radeon 4870 1GB
Radeon 4870 X2
and if your budget is tight 9800GX2.