Lol, if you want a GTX 460 you will definitely need to upgrade.
Well you know what cpu you want, look for a board that supports your cpu (socket am3) with good solid features. And of course try to aim for a reputable brand (such as Gigabyte, Asus, MSI, etc.)
For ie. i pulled up a list:MSI AM3/AM2+/AM2Cooler Master 650wGigabyte GTX 460 768mbAsus GTX 460 1GB
Now the board is a mATX form factor - meaning it's smaller than your typical motherboard (ATX form factor). This is based of your current motherboard in your system. And as long as you install the same form factor of a motherboard, there should be no reason why it won't fit in the case.