Re: 9800GTX or GTX280
The gx2 is not that great as it only has 512MB of VRAM and will struggle with AA. You'd be much better off with a 280, 260 or one of ATI's 4-series cards like the 4870 1GB or 4870X2.
If you're sold on multi-GPU cards, the 4870X2 is where it's at. For the strongest single GPU card, look to the 280.
i7-2700k @ 5.0 GHz
Nvidia GeForce 570 2.5GB Tri-SLI
Asus P67 WS Revolution (Tri-SLI)
OCZ Vertex SSD x 4 (Raid 5)
G.Skill 8GB DDR3 @ 1600MHz
PC Power & Cooling 950W PSU