I am now noticing my 7800GTX cannot run full settings on even older games with the new monitor (1900x1200) never mind the new ones with all high settings. I can get an old used 2nd card 7800GTX for SLI but that seems sort of silly for old tech. What should I do? HD4870 ?, GTX260? For a 24" monitor what a good upgrade from a 7800GTX single card? I got a silverstone 640 watt zeus PSU so I should have enough power.
It would be better if you could share some more about your rig (processor, MB, RAM) and what kind of games you're planning to play to get a better feeling on what to recommend.
Most likely a 4870 would fit you best, but this is not the only option. Given your resolution, you might think on SLI (assuming that you have an SLI capable board). If you don't have an SLI/Crossfire board, you still can have a 9800GX2 or even the new 4870X2, depending on your pocket size.