thanks everyone. I actually am taking my ugrades process a little more seriously now because I have to hold off on my Macbook Pro 17" until the summer/fall (its hard to get student loans big enough for both tuition and a laptop). So I'm gonna use money I have in hand to upgrade my tower so I have more computing power in my dorm as I don't really need to do as much intense work on the run as I would at my desk. so heres the plan:
CPU
http://eshop.macsales.com/item/Newer%20Technology/MAXG48S1800/
$ 359.99

SATA Controller
http://eshop.macsales.com/item/Firmtek/SATA1V4/
$ 119.99

Hard Drive (x2)
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148337
$ 129.99

SATA Cables (x2)
http://www.newegg.com/Product/Product.aspx?Item=N82E16812270184
$ 5.99

eSATA port bracket
http://www.newegg.com/Product/Product.aspx?Item=N82E16812226006
$ 2.99

$ 754.93

this is all for a QS tower that I got for 50$
I plan to keep this computer for a very very very long time seeiing as when I finally do get my Macbook Pro in the fall, I will need a Mac OS 9 capable machine for working with Forge and running old school software/games that have yet to be ported to OS X.

what is your input?

also How do you think Doom 3 would run on this rig?
(My video card is an ATI Raedeon 9200 Pro 128MB
and I have 1.25 gigs of RAM)