280 GTX sli (285)> 295 GTX > 4870 X2
4870 X2 > 295 GTX > 280 GTX sli (285)
the 280(285)s will require a motherboard change, it doesn't need to be tri sli, but it does if you want to eventually put in 3.
If your computer matches the specs in your members configuration page, none of these card setups will work in your computer, and if it does, the CPU will be right there to bottleneck.
I would suggest either upgrading CPU first, or grabbing a less poweful card.
What resolution do you play at?