Memory support in all athlon XP/MP processors is a function of the supporting chipset.
while there are many chipsets supporting the XP, only two support the MP in multi cpu mode, ie the MP and the MPX chipset, both made by AMD.
neither of these chipsets support RDRAM.
for the XP it is always possible that someone may produce a chipset to support it, i dont know of any that have yet.
RDRAM support for CPU''s seems to be a project restricted to intel, due to there close ties with RDRAM, ie they have invested large sums of cash in RDRAM, and only there 820/840 p3 chipsets and some of there p4 chipsets support RDRAM.
RDRAM did have its advantages when the p3 was in its heyday, however with DDR2 now upon us and talk of DDR3 i think its advantages are less prominant.