DDR for athlons really doesnt cost much more and there is a speed increase, there are just several things yuo need to make sure of.
1: get an amd 760 based motherboard
2: get pc2100 ddr from crucial.com (115$ for 256mb pc2100, pc133 cas2 is only 7$ cheaper so price is really not a factor)
3: since you are getting a 760 based motherboard which supports a 266mhz fsb make sure you get an athlon that has a 266fsb that will help performance even more.
for the motherboard i would reccomend the asus a7m266 has agp pro as well.
the downside of ddr is most boards only have 2 slots, and i am not sure of any 512 pc2100 modules exist so you would be limmited to 512mb of ram and that may not cut it for you.
I was waiting for the amd 760mp chipset to come out and it never did so i got dual 933 intels with pc133 its very nice but i am waiting for dual 1.5 amds, i would stay away from the p4 rdram is expensive and the performace of amd chips sdram or ddr are better if you want to stay on the single proc route.
nick