This may not even be possible. You mentioned that it's a Gateway. I don't think Gateways conform to any standard (such as ATX). So it may not be possible to find a motherboard to fit your case. You do not have to worry about the speed of the RAM. Generally speaking, faster is better. If it's...
If you're planning to upgrade your CPU soon I would just wait due to the prices of RDRAM. Any CPU you upgrade to will not use RDRAM. If you decide to wait on your CPU, I would purchase 2x256 modules to have a total of 768MB.