I am trying to answer your queries one after another.
1) Which is better performance wise...adding a second BFG GeForce 9600 GT OC and using SLI to combine them or replacing the 9600 altogether with a single EVGA GTX 275 OC? How much is the difference in real throughput? I know the difference in money to me is about $140 for an additional 9600 GT OC versus $260 for a GTX 275 OC.
A: A single GTX 275 will definitely give better performace in comparison to Geforce 9600GT SLI. It will give about ~22% boost in performance. You can see some benchmarks:
http://www.techpowerup.com/reviews [...] 0/13.html#
http://www.guru3d.com/article/gefo [...] ew-test/10
If you want to spend $120 more then go for GTX275.
2) I'm pretty sure the 650W power supply on my son's computer is more than adequate for a single GTX 275 OC, but if I were to go with a second 9600 instead, would his power supply handle this second graphics card adequately with the CPU he's using?
A: Yes, 650Watt power supply would be adequate for 2x9600GT. Is you PSU has 80+ rating? If you know the approximate wattage for your system PSU then this is the link:
http://www.msi.com/index.php?func=power
3) If I did replace the 9600 altogether with a GTX 275 OC, I'd like to put the 9600 in a Pentium 4 based Dell Dimension that has a 305W power supply and use it as a cheap gaming server that basically just needs enough graphics to run programs, but not run them at top speed. Would that power supply, although likely undersized, handle the 9600's load in addition to a single hard drive?
A: The recommended PSU for a 9600GT is 400Watt. I think it will choke the current PSU of Dell Dimension system. And I don't think 305Watt PSU could not give more than 70% throughput.
4) If the best choice is to replace my son's 9600 with a GTX 275, could I add the 9600 as a physics GPU in my computer, thus having (2) GTX 275's and a singel 9600? Would my 1000w Corsair PSU handle it? I assume the configuration would be the two 275s SLI'd together and the 9600 would sit in a standalone slot?
A: I would suggest to go to the link for the Wattage of the PSU:
http://www.msi.com/index.php?func=power
Could you please provide your System Configuration?
Hope this will help.