I would definitely recommend building your own computer. It can be done for MUCH less that 3500$, at least 2500$ could go to more audio equipment[].
I would recommend: (prices are just rough estimates)
Intel Core 2 Duo (Conroe) ($200)
at least 2 gb of DDR2 RAM (fairly inexpensive at this point) ($200)
again a reputable motherboard ( PCI express compatiable IMO) ($100)
decent graphics card (x1600pro, 6600gt, etc.) ($100)
and raid 0 (twice the speed), 2 new Perpendicular 250 gb drives ($160)
That would be a great system for a long while if taken care of, most of the time systems get conjested with all the random crap out there even if they are taken care of, I do what thepathlesstaken recommends where 10-20 GB is used for simply the OS and all your documents are on a seperate partition, so i can reload whenever without taking a long time to do so, a fresh OS is always very nice. However if you just want to improve performance of your current system i
recommend running "msconfig" and going to startup tap and unchecking
some things you know you dont want starting immediately, bc these take
RAM and take forever to load on startup.