Running 7, yeah 2 bare minimum, 4 much better, I run 6 with 64-bit version.
PSU wise, I'd go 500-600W so you can salvage it to next PC you buy possibly.
Thermaltake and Antec are good names that probably have something in your price range at those Watts.
If you are upgrading that PSU then I might try a better video card, 5570 is pretty bland, that 2nd 5 is the indicator that it sucks to be blunt.
Plus the new PSU has the 6 pin connector you need for a higher card.... it's just now the thickness of the case that may come into play, I have an expensive Mid Tower, even so my 5850 was really a tight squeeze.
Here's a video card example:
Newegg.com - VisionTek 900287 Radeon HD 4850 1GB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFireX Support Video Card
That would make mincemeat of a 5570 is all tests, 256 interface! and 8 for 2nd digit rule in effect.
Price is higher, but you really get what you pay for and worth it if you wanna game juicy.
I'd try a local store that lets you return stuff, since size matters here, possibly. :)
It's worth the extra insurance policy payment and helping local economy.
See this review on the 5570 says the 3650 is a better card in the conclusion.
ATI Radeon HD 5570 Review - TechSpot