I used to use an AMD chip, but left them when Germany began stabbing us in the back.
An FX-53 is about $800, with the 939 pin being a tad more. If I am going to spend that kind of cash, I will dump $200 more in and get the 3.4EE with the 2 MB level 3 cache, in the 775 pin form(the 925X chipset, when available).
I might look at AMD's 64 bit systems when Windows 64 comes out in retail version (its only a beta right now)though I'm not sure the 64 bit is going to be as hot as some people say.
You should get quite a boost from experiences already had with UT on 64-bit Linux. From Tim Sweeney of Epic games:
The extra registers [actually, twice the registers] can give you a significant performance gain in loops which currently have to spill over into memory. The on-die memory controller reduces the path for uncached reads significantly, which is a huge win for applications like Unreal which are memory-limited. On the other hand, pointers grow from 4 bytes to 8, so traversal of linked data structures takes an additional bandwidth hit. I think you'll probably see a clock-for-clock improvement over Athlon XP of around 30% in applications like Unreal that do a significant amount of varied computational work over a large dataset.Going 64 may not help with word processing, but it apparently will help quite a bit with these demanding games. This is an AthlonXP vs. 64 comparison, which counts an architecture that just in general gives at least a 10% improvement, but you should see maybe a 10%+ improvement going from 32 to 64 on the same AthlonFX chip.