Justinkea,
I've had only one AMD system, an Optiplex 740 that arrived with an Athlon X2 64 3800+ (2.0GHz)which I changed to a 6000+ (3.0GHz) . the jump in clock speed is very similar to the one you're considering going from 2.1GHz to 3.2Ghz . I found the upgrade to be worthwhile as I paid only about $20 for the new CPU, the speed improvement was noticeable, and I didn't want to go through the trouble of reloading everything.
In general I'd say go ahead- it's a quick way to see a more responsive system, but whether it's worth it only you can answer. The thing is, a used 6400+ is about $60 which may be a fairly high percentage of the value of the system. You might- just to make sure- take the value of your current system plus...