IsOs, Faster, high quality RAM is the trick with SETI: high FSB speed, BIOS settings for CAS2, 4-way Interleave and Turbo(Via chipset), etc.
RC5 is small enough to fit into cache and memory speed doesn't matter. That is why I have all my Intel systems with 66 MHz FSB running RC5 and have all my SETI stuff at 100 to 133+ MHz FSB systems.
I have found that the minimum amount of RAM for SETI, to all MAX BIOS memory tweaking, is 128MB (2 x 64MB modules). For RC5 either 32 or 64 MB under Windows will be fine, assuming the box is dedicated to RC5. I have 256MB of RAM on three boxes that get used for lots of other things. The dedicated boxes all have less. So, putting more RAM in the box will only help IF you are running other tasks that benefit from large amounts of RAM. An example might be PhotoShop where you are manipulating large scanned images.
Then again there's bragging rights. How many folks can say that they have 1GB of RAM?