SLI and Crossfire configurations are always a bad option. Few games support them to their fullest.

Unless you're planning on playing on a resolution higher than 1920x1200, don't bother with SLI or 1GB (single) cards.

A single HD4870 will run any single game out there right now at its maximum settings. It can even run Crysis with everything on high/very-high @ 1680x1050 @ 30+fps

I'd recommend a completely different setup. Get:

Intel C2Q Q6600 + Zalman 9700 heatsink

4GB of DDR 800 RAM

Gigabyte GA-EP35C-DS3R mobo

650W PSU

Radeon HD4850 (Gigabyte, MSI, Sapphire are great brands)

Windows Vista Home Premium 64-bit

You'll save more than $200, yet receive the exact same performance. Trust me, there is less than 1% performance increase between DDR3 RAM and DDR2 RAM.