Because you can turn up the graphics settings (ie AA/AF) because that is all done on the GPU - no fps loss, but better quality.
3700's overclock like MAD if you get the right one - some hit 3 GHz on air (Stock - 2.2 GHz, remember!)
A 4400 is dual core, so its actually two 3-somethings strapped together - the fastest single core AMD non-fx CPU is the 4000, but a 3700 is a better bet because they overclock a LOT
If your going to overclock, then get an Opty, specifically a 165 as its dual core, and you can still get 3 GHz out of it!