How much have you got to spend? Do you own a copy of windows that didnt come with your computer?
A cheap AMD upgrade would be under $200 for a motherbd $60, cpu $90, and ram $40.
If you don't own a copy of windows then you'll need another $100 roughly since your oem version wont run on the new setup. They are usually tied to the bios of the OEM motherbd. Other than that you can reuse your drives, keybd, mouse, & monitor (and graphics card if you have one)