you'll need a new motherboard ontop of your new processor. and new mobo means new ram because you're probably using SD RAM.. and now only ram in use is DDR and DDR2.
and since gateway uses propriety parts (i think) then you'll also have to buy new case/powersupply.
you'll pretty much need a whole new computer.. you can keep your hard drive , OS, and optical drives.
depending on the quality of parts you want.. you'll prolly be looking to spend around $200 - $400
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius