One consideration is what system you have and how you use it.
If you currently use a resource-hungry OS like XP, I would suggest the 512MB setup.
As soon as you need more than 256MB RAM, you are swapping data to and from the swapfile, which is effectively a temporary simulation of additional RAM, with one big drawback - it is held on the hard drive.
If you can keep all of your running processes within 256MB, then the 800MHz setup will be quicker. However, nowadays this is unlikely to be achievable with the typically large memory-hungry software that is the norm. Therefore, 512MB of slower memory is likely to run the system quicker than 256MB of faster memory.
There are many resource-monitoring tools that can tell you how much RAM you are using / have used so far, as well as the Windows built-in tools.
I have never used rdram, so the above must be seen as generic guidance that is equally applicable to PC2100 vs PC2700 memory, etc.
just to add: i have all the memory in at the moment = 768mb rdram.
just wondering really whether i should try to replace the 600mhz rdram with 800mhz rdram - or whether it really isn't worth the bother - i was just thinking there was no point in having an extra 512mb ram, if its slower speed would diminuish the performance of all of the rdram