Probably more than just a new motherboard and RAM. The video card, hard drives, sound card, and many others can also be included. Chances are, if you upgrade the CPU, you'll also have to upgrade the motherboard and RAM as well. If you've got an AGP video card, you'd likely have to upgrade it to the PCI-express standard (as AGP is expensive and unavailable on today's systems). PCI (non-express) still survives and is rarely used now.
Edit: also, if you're using an OEM version of Windows, you may have to buy another copy of the OS. If you have the retail version, then there's nothing to worry about, provided it's a legal copy.