Uhm... To match 2 9800GT's in SLI in things that are not DX11 (like WoW, UT3 engine games and XBox ports, mostly) you're very limited with a single slot option... I mean, to consider it a worthy update on your current platform. Basically, having Direct X 11 for gaming alone doesn't justify changing 2 9800GT's yet IMO.
To be honest, focus first on the CPU (like 4745454b said); maybe get a PhII X4 955 C3 stepping and run it at ~3.9Ghz or directly get a PhII X6 (Thuban) if your MoBo supports it. That will give you a few extra FPS's with DX9c (most games anyway).
Unless, that is, that you're also looking for other new features... UVD3, HDMI 1.4 support, Sorround or Eyefinity, etc; or the specialized new DX11 features: tessellation, SM5.0 and DC (the multi threaded capabilities are also supported in DX9c - DX10.1). In that scenario, a 6950 would be the best Price/Performance video card for your system IMO. It would be a worthy upgrade from 2 9800GTs and perform a lot better (or give enough extra FPSs to justify it).
This is my motherboards official page with full specs... could you tell me which CPU is better,
6-qore and quad-core cpu to choose one of them, depends from the money that i will have...
I don't know from AMD CPU's and which family is better, i was using intel all the time, and I change it just to try, when i was building this computer....