Motherboard replacement question

Hello, I have a 2 year old Alienware box, which currently has a motherboard which has only AGP slots.

I am looking to upgrade my graphics card for more up to date things, such as oblivion, etc.

My question is this, is there a way for me to buy a PCI-E motherboard, and simply replace my current intel CPU into the new motherboard...

or are the actual CPU's "one shot" deals?

also, are there any other complications to think about?

I am just wanting to pick up a nice mid level PCI-E gfx card for a couple hundred bucks or so, and if there are going to be issues i dont forsee, ill start looking at other options now,

thanks for the advice!!