i have a e8400@4050mhz 4 GB OCZ titanium runs 4-4-4-12 @ 800 5-5-5-15@1066 EVGA 680i SLI motherboard PC Power & Cooling 750 Quad 36 raptor (OS) 150 raptor (apps) 320 & 500 GB WD 7200 rpm drives
and i have a 9800GTX with 60 days let on step up. i had a 8800GTS 640 but the girlfriend wanted to play AOC too and her 7900GT 256 SLI set up was a little choppy i am assuming due to the lack of framebuffer.
so the big question is like the OP do i SLI another 9800GTX for $269 or spend $350 and step up to the GTX 280?
the lack of memory on the GTX's is my main concern
generally i play AOC @ 1680x1050
so what to do?
GTX 280 was shown to be beaten or matched most of the time by 8800GTs in SLI, so I'd have to say go with another 9800GTX. Don't spend more money than you have to and you'll wind up with better results.
EDIT: In my experience anything 1900 (1080p) resolution and under, 512 megs should be sufficient, the 9800 GTX should do you well, and you have the option for tri-sli:)