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.
There have been some issues with the PCIe 2.1 cards in older PCIe 1.x slots. I don't think gateway has bios updates to fix these issues if the exist on their boards. To avoid the issues I would suggest a HD4670 which is a 2.0 card.
Its lookin good for the GT 240 then since its also a 2.0 card. It gets better benchmarks than the hd 5570, and has a better chance of working with my mobo, and I can even get it for a little cheaper. The only thing is power consumption, but even if its draws a few more watts, its still low power and hopefully my power supply can handle it.