Your power supply is not powerfull enough to run either of those cards. Here are the minimum requirements for each card:
8800GS - 400W PSU minimum with 26A on the +12V rail.
9600GSO - 400W PSU minimum with 26A on the +12V rail.
The best card you can run on a 300W PSU in my experience is the 8500GT which is a pretty bad card for gaming but it is very cheap. With your processor it would not be advisable to have an uber powerfull graphics card because it will be bottlenecked by the CPU.
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