So, given that 4870 < GTX 280... and GTX 280's performance...

[Jax Omen]

I already have an 8800GTS (G92). Should just get a second one of those for some SLI?

I game at 1920x1200. So far I've had to knock settings down from the max on Crysis (DX9 or DX10, doesn't matter, although obviously DX10 goes down more), Lost Planet (DX10 - I can max DX9), and Supreme Commander (CPU limited).

 

[Snakexor]

I would say either get another g92 gts or get an hd4870 or two and xfire them. However, with your standing mobo, the g92 sli is probably the best choice.

 

[happy medium]

Yes in my opinion thats the best bang for the buck right now.
2-8800 gts's should beat a gtx280

 

[extra]

Seems like, logically, that would be the best option for you. (to get another one and SLI it).