You really need a larger power supply to run better graphics; it is really essential. While the 5570 only uses 45W on full load, remember that that PSU also has to give power to the motherboard, processor, hard drive, and anything else is in the computer as well as the stuff you have connected to it, such as keyboards and mice as well as MP3 players. A decent power supply is imperative and also not that much of a stretch for most people's wallets.
That 5570 does look like a nice card for the case since it is low profile, but don't expect to play ANY high end games on it unless your monitor is small or you play on ultra-low.