Well you've first gotta determine what your computer will be used for.
If its mainly for gaming, and you don't do to much video editing, or music encoding you should go with the AMD line.
Also, what kind of Ram do you have? DDR ###?
But.. here's the thing.. if you were going for future upgrade ability I'd suggest the FX-53 939 Pin. This isn't a cheap processor though.. probably the most expensive CPU out there ($800+) for AMD. (Then again that CPU is top of the Line)
Now if you want a good cpu that will do the trick I'd suggest the AMD Athlon 64 3400 or 3200 (402/270 respectively). They use socket 754, and you've got 2 upgrades if you so choose in the future the 3700/3500, they all use "unregistered" ram, which is faster and cheaper than "registered" ram. The 34/32 give good processing speed with a big 1 MB L2 Cache and a nice bus speed.
AMD "says" they're only going to continue researching/developing the FX style processors (939/940). But most computers are outdated within a year, so this wouldn't be a big thing if you went with the socket 754's.
As for motherboard there are a few options you can go with. Just depends on your chip.
