RDram or AGP

config:
p4 1.4 on intel850 mobo
640mb ram(2×64 pc800 RDram;2×256 pc700 RDram)
16mb nvidia gcard

my comp seems faster with only the 128 pc800.
since my mobo doesn support pc700,all of em(including pc800) are working at 600mhz when i put in the pc700.

i have an option of getting a 256mb AGP card instead of the 512pc700. should i go for it or try overcloacking the under performing 640mb rdram?