Do you have a custom build? From what I understand, you currently have a motherboard with integrated graphics, and you'd like one with a PCI-Express x16 slot so you can add a dedicated.
There are motherboards of all prices, to help us figure out what's best for you it would be nice if you posted some of your specs so we can find what will fit your current parts. If you don't know this information and you own a brand-name computer such as Dell, then I'm sorry to inform but it might be impossible to get a new motherboard without buying a new case and maybe a PSU.
edit: Dang it Ace got me beat.