I was using an "off the shelf" Dell which I initially bought with Maya (or at least Flash or other 2D animation) in mind. It worked OK with earlier versions of Maya, but 6.5 made the ol' warhorse crash and burn! Earlier this year, I was compelled to retire the old warhorse (actually, it's still in use) and I decided to build a new PC from the ground up.
The first order of business is a motherboard which can hold a LOT of RAM and the latest processors. I chose one by Gigabyte Technologies which holds up to 8 GB of RAM ( I presently have 2 GB, all I could afford! :-) ). I used an upper mid level Intel 3.2 GB processor, and chose an ATI DirectX 9 compatible video card (which, BTW works great).
I had some "fun" building it (I bought all the components from newegg.com); some of the items didn't work right the first time out, one motherboard was dead in the water when I got it (quickly replaced), and I was advised to buy the wrong kind of RAM (choosing compatible RAM chips is a black art, but eventually I got it right). Finally, it worked!!!!!!!!
My only (and by far BIGGEST) mistake was choosing Windows XP 64 Professional as my operating software. As I learned, it was virtually obsolete upon release, and while it works GREAT with Maya (very stable), it is virtually UNSUPPORTED by just about everyone else! I had to unretire my old Dell to do all of my scanning and printing. BE ADVISED...NEITHER Symantec nor McAfee support 64 bit windows! I lucked out and found a small UK company called "Avast" which to my knowledge makes the only anti-virus software compatible with Windows 64. I have found that software easily the equal and in many ways the superior of Norton and McAfee! When the new Windows software comes out, I may wind up getting it.
If nothing else, upgrade your motherboard and get more RAM! Otherwise, I found building a new PC an interesting challenge, and you can buy as much or as little as you need!
"Ad astra per aspera..."