Not responding and low virtual memory

[hev]

I have a Dell XPS from 2006. It still runs decently but I am having the above issues, way too much. I thought maybe I can upgrade some hardware for now instead of buying a new one.
Definately RAM, but what else?
My hard drive is using 78gb and had 158gb free, so I was thinking I don't need to upgrade that.

I just remembered, when I restart, a lot of the times it goes through chkdsk.

Oh, and using XP, which I prefer to keep.

So suggestions are welcome and appreciated, thanks.