Firstly, thanks for your unfounded input sabot00.
Your question is a tough one, and you really need to think about the benefits of each card.http://gizmodo.com/5467822/ati-radeon-hd-5570-review-and-benchmarks-reasonable-gaming-dx11-performance-for-80
While this bench is done on the older 9.12 drivers for the 5570, there wouldnt be a lot of difference to now i wouldnt think. The gtx240 smashes it in almost every test, with only one problem...DX11. Since you wanted to play your games nice and pretty like, DX11 is a must, and the 5570 is better for that. Unfortunately, i doubt you'll be able to play SC2 or Diablo 3 at max with the 5570...which is why i suggest the 5670 at the least. DX11, and much better performance than both the 5570 and 240 and for a measly 30-50 bucks more.
I must go, work calls, but i will try and get back on to see about your power supply.