Is this a good low power card for gaming? Questions?

I want a graphics card that has the capability of playing most modern PC games on AT LEAST good medium/standard graphics settings if not high quality settings. It should be compatible with DirectX 11. The max I'd likely spend is around $120 before shipping. I'm going for the BEST graphics quality I can find in that range that will still work with my system. My main obstacle is a small power supply. I have:

300w PSU
AMD phenom 8450 triple-core CPU 2.10 GHz
4GB DDR2 RAM
Two physical SATA hard drives (total about 1.6TB)
An LCD monitor with a native resolution of 1280x1024
A few devices without a separate power source like DVD burner, media card readers, camera and other regularly used USB devices
Windows 7 ultimate 32-bit OS

I've read Radeon cards are not very power hungry compared to nvidia, and 7700 or 7750 are some better ones that may work with a small PSU. Here's one I was looking at; would it work?

http://www.newegg.com/Product/Product.aspx?Item=14-161-...

Are there better Radeon GPUs that could work with my system? Does the card's brand matter when considering performance/power usage also? If so, what are the "best" brands that are as affordable if not cheaper?

Since I'm not familiar with Radeon I'd also like to know if it's common to encounter game/software compatibility issues with these cards where nvidia cards wouldn't? Especially considering I've noticed some games that list only nvidia cards for system requirements. Or does Radeon have the exact same capabilities as an nvidia equivalent while the only difference is the brand name?

I want suggestions for what WILL work with my system, not "might". I don't want to lose money or wasted effort on returning something. Also I'm NOT looking to replace the PSU. Post the most specific suggestions you can, please! Thanks!