I have a lowly GeForce 6600 (PCI Express), so no extra power needed. But I still have stonking graphics, running FSX DeLuxe at all Ultra-High settings. I think it is not just the graphics card you should be thinking about, but perhaps improving your CPU and RAM for improved graphic capabilities.
The Radeon 3650 might be a good choice. They use very little at all (they don't need additional power) and have all the latest features. It's slightly newer than the Nvidia 8600GT, and the benchmarks also suggest it's a little faster.
What about a 7600GT? Great PCIE card, can be found cheaply. You are right in thinking that the 8800GTS requires a seperate power connection, and your PSU would not be up to the job of running it anyway.
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