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.
dell precisiom 390 supports only some model of quad core processor not all so it is better to save money and get a newer system like i5 at least then a normal level gpu will give you a very good performance