I recently upgraded from old 64mb of RAM to a newer 256mb of RAM. Can I run both of them together or is it only possible to run one type? I put the 256mb in and left the old 64mb's out and it seems to be running fine now.
I have a 750Ghz Pent.II with 30 Gb of HD, 8x CD, and running Windows 98.

My question is, can PC100 memory run alongside PC800 memory or will the old one slow things down?

What RAM you can use in a system is dictated by your motherboard. This will be detailed in the documentation for the motherboard. It is possible to use both modules but its also possible it will slow it down. Owever, I question whether your new memory is running at the faster speed anyway. It will run at the speed of your bus. If that was 100 then that's what it will run as.