Thanks for the comment people, let's hope that 2 256mb's will work.
Oh, one more thing. If you keep the two 64mb’s installed and add two 256mb’s and you are using Windows ME or 98 you will have to adjust the VCACHE.
Is it actually possible to have both the 2 64mb's and the 2 256mb's together? I've never seen a PC with 640mb of RAM.