[Build help] Thoughts on the Radeon 4550?

I'm in the market for a graphics card, but not necessarily an amazing (expensive) one. My hope is to use this computer primarily for gaming (LoL, Magicka, Dota 2, ect). The Radeon 5670 is $75 at Best Buy, and is recommended by Tom's Hardware, but the Radeon 4550 is on sale for $50. What's the main difference, and is it worth the extra $25? Thanks in advance!

Edit: My motherboard is an Asrock AOD790GX/128M, if that makes any difference. Not terribly amazing, but I got it for free :p