Depends on your PSU, if you PSU can handle the GTX 460, get t.
If not, get the HD 6850, it is better than the GTX 550ti and a bit faster at stock clocks than the GTX 460, and only uses 1 PCI-E 6pin connector, same as the GTX 550ti.
For your resolution even something like the HD5770 would be very good. If you do go with the GTX 460 just get the 768mb model and save some money. You will see little difference at that resolution compared to the 1gb card. 600w on a good brand like Antec is more than you need really. If an Antec 500w can save you some cash then go for that as it will be more than enough for the cards you are discussing.