1) SLI: buy two GTX260s, each one for 10.5k. Model: Palit GTX260 Dual Sonic. Since you want to go for SLI setup , it is very important that each card stays coola nd Palit sonic editions with dual fans are perfect and this is one is the best card you can get for that price. Also you can check GTS250 for 7k
2) Crossfire: ATI HD4770, each one for 7.5k. This one is 40nm fabrication based card and runs damn cool and two of these can outperform GTX275.
3) Go for 64-bit windows 7 once it gets released in October and you can have 100Gb of RAM too
4) first, dx11 is gonna take some time to get into developers minds. The big advantage with dx11 based cards is not that they support dx11 but in the architecture
They are based on 40nm process that makes them lot better for overclocking and hence lot more stable.
They are much more power efficient and require less power when compared to existing set of cards
They are going to easily outperform current gen cards be it price or performance or temperature