Upgrading an old pc

I have a few questions about upgrading my old PC.
I want put a bigger HD and upgrade the RAM and was wondering about a few things. I’m currently running W95 and I simply want to replace the current HD instead of slaving a new HD and installing 2000pro. Just wondering how do u find out which HD and RAM is compatible with my motherboard(it’s dell demesion xps h266)? Any input would be grate.

thanx