Dual 6950 on a Dell Precision 390

Hello and Welcome.
You can't CF the HD 6950s for several reasons, you might have a Dual Core CPU which will bottleneck the HD 6950s performance in CF, so you're better with a Core 2 Extreme Quad Core CPU. You have a 375W PSU, you need at least 650W PSU certified for CF/SLI configuration. The most important limiting factor, you have a PCIE x8 slot routed as X4, this will affect the performance of the GPUs.