Dell gx280 motherboard upgrade to Dimension 9100 motherboard

Hi all!

I think I chose the rite section for this post (At least i hope it is.)

I have a dell optiplex gx280 sff. Windows 7 ultimate 32bit, 3G ram. which i have managed to move to a full sized E-Machine tower and successfuly incorporated the power and front USB casing pretty well. Next is to upgrade the video card (Cheaply)

But first off, and most importantly. I'm trying to find some way to incorporate a dual core processor into my system, so that i may play my favorite game - GTA 4, which requires a dual core processor to play.

I've seen online that the gx280 motherboard does not support any dual core anything.

I have found some one who has a 2005 dimension 9100 complete with Windows XP, Bios A01. When I checked it out online I found that it's motherboard supports a dual core processor. But i can't find anywhere that said whether or not it is compatible in all dell computers as long as you make it fit or not.

Will any dell motherboard fit, or am i stuck with a select few. And even then my power supply is rated at 310 max, does that need to change to? It also runs a 64bit system, what do i do with my 32 bit windows 7 disk? Will it recognize all my other components? how does a motherboard work? What does it matter when you switch motherboards, as far as I'm aware the motherboard is the whole systems brain, if you change it, Then it's all gone. What do i need to do to make this happen?

I'm pretty good with figuring things out on a computer OS system, but when it comes down to hardware i am clueless!

Really all i want is a dual core processor. as cheaply as possible.

Any help, Any one, Please.

Thank you in advance.