GTX 260 Core 216 SLi is still viable, particularly if you OC. Two of those cards @ 650|2300 or greater are nice.
Another card would yield a nice performance boost, particularly if you game @ 1920x1200.
HD 5850s are around 300usd, a second GTX 260 could be had for around 175usd.
There's one that's 650|2000 out of the box for 176usd
It does have a 216 core. At clocks like that you'll definitely have faster than GTX 295 performance.
GTX 295 is 576 on the core.
EDIT: I tried the Heaven bench on my GTX 260 SLi rig with the cards set down to 650|2000. I did the DX10 path because tesso doesn't work on these cards. @1920x1200 with default filtering they did 70FPS. At my normal OC they do 75FPS. My HD 5870 @ 1030|1300 turns in 66FPS under the same conditions.