OK, in a nutshell, yer options are (cheapest first):
- new 7800* AGP card (or whatever the best ati agp card is).
- new 939 mobo, 7900* / X1900* PCIe card(s).
- new 939 mobo, new 939 CPU, 7900* / X1900* PCIe card(s).
- new AM2 mobo, new CPU, new DDR2 RAM, new PCIe card(s) OR new 775 mobo, new CPU, new DDR2 RAM, new PCIe card(s).
Personally, i think it's a good time to get a cheap 939 mobo and new video card(s). Most games are GPU, not CPU, limited, so you'll get the best performance increase / $ with new cards instead of new CPU.
The cheapest option, new AGP card only, is also quite plausible, because as there's not much difference between AGP and PCIe performance for the same chip, the only reason to get a PCIe board is to use 7900* cards that don't come on AGP, or to use SLI/Xfire.
I'll assume you know how to overclock the crap out of yer FX53, so upgrading to an AM2 or Conroe will see a bit more performance, but not worth the $, IMHO.
And getting a new 939 mobo and a new CPU are probably not worth it, either, just to reuse yer old RAM (unless you've got 4gb of 2-2-2-5 or something).