Go ahead and splurge!
Get the new motherboard, RAM and CPU like planned... Also get a case with power supply, a hard drive, DVD-RAM, and a KVM switch like this:
http://www.newegg.com/app/viewproductdesc.asp?description=17-107-407&DEPA=0
That will allow you to use your keyboard, monitor and mouse on both machines... switching between them with a double-tap of the 'Insert' key or something like that.
Use the older machine for downloading, email and web browsing etc. Use the new one for playing your games and running your other apps. They can share folders when networked so you can move stuff you download to your new machine.
Just a thought.

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