Here's my recommendation. Stick the gts 450 back in the mac pro, sell it and build yourself a desktop. since you already have a good gpu you can spend the money you get from the mac pro on everything else. I would recommend the following:
Case: Coolermaster HAF 922
Motherboard: Gigabyte z68a-d3h-b3
Ram: 8Gb ddr3 1333mhz
Cpu: Intel i5 2500k (overclock it to 4ghz to alleviate any possible bottlenecks and it'll improve fps in games like skyrim)
Psu: 650Watt Modular psu so you can do cable management
Fans: 1 280mm top mounted fan (fits the haf 922), 1 120mm sidemounted fan.
HDD: id recommend getting a 64gb SSD for the operating system, and a 1tb for games and music.
You can pull out a dvd drive from any computer sitting around.
Also, yes it could be the psu. Just because it's rated for 900 watts or whatever it is doesn't mean it's necessarily "pushing out" that many. the psu could be shot.