Continuing with Fitz's comment...
How much ram do you have, what type, number of sticks?
What type of video - on board, video card, etc?
What is the rating of your power supply? It may limit your options as much as your MB.
The best upgrade recommendations depend on what you have now. Like if you only have 1 gig of ram the best overall improvement may be to add more ram.
If you are into gaming the recommendation might be different than if you are into video editing or graphic arts.
So give us some more info and we'll help you out. Spending other people's money on upgrades is our favorite pastime around here.
If you don't know the answers to these questions you can download CPUz.exe from many sites (I use and trust
www.majorgeeks.com) and it will read you hardware info for you.
Uh yeah, and welcome to OSNN.