^ I'll second the sli, but the 4870 in CrossX scales better and outperforms the 280 GTX in alot of games surprisingly (or comes close).
The 9800 GX2 is a good option, but it can be a hassle when the game isn't full on par with sli requirements (GRID wasn't when it was released).
They are sold really cheap now though, here they are around 399.99- 450. And on Newegg I saw a couple for 350$
2 9800 GTXs will slightly outperform 1 GX2, (some games the GX2 has the upper hand since it tends to scale better in single slot.
The only hassle you'll have is with Quad SLI, I've only seen the GX2 shine on COD4 to make a fuss over it. It beat 3 280 GTX in tri-sli and in Reg-sli.
Other than that the GX2 seems to have problems with most users, my friend says its just rumours for most games, but it holds its own...but I'm sure you don't want to risk it.
My ultimate Suggestion would be wait for the 4870 X2. 1 of those cards will be stronger than buying a 280 GTX or buying 2 260 GTXs, and also cheaper for now.
If you don't have a CrossX board, I still think it would be a good solution since, well its already sli
(rumored to work as a single GPU).
Thats my 2 cents
P.S
I think if you really wanna go cheap, I'd look into the 8800 GTS G92s as well (alot better than 2 9800 GTXs for P/Performance