Exactly as i said, add another gig, if you only have 2 slots and there used then just take out one 256mb stick and replace with the 1gig stick, then you will have 1256MB's of ram should make a fair bit of difference

And yeah as long as its the same speed/type of ram it will work fine.

Go onto crucial.com and they have a system scanner, it will not only tell you what ram you have but it will also advise on whats best