pc1066 RDRAM is still a bit faster than most DDR, with the exception of when it's DDR on canterwood boards. But consider the price:
RDRAM
2x128mb pc1066=$100 (insufficient for decent gaming on XP)
or
2x256mb pc1066=$230
or
2x512mb pc1066=$475
DDR
2x256mb pc3200=$90
or
2x256mb pc3700=$125
or
2x512mb pc3200=$160
or
2x512mb pc3700=$200
or
2x512mb corsair pc3200 twin pack, tested together in dual channel=$283