listen to what shue and mojo said. buy it all at once, put it together all at once, and get USE out of it from day one. remember what weird al says in "it's all about the pentiums"
My new computer's got the clocks, it rocks
But it was obsolete before I opened the box
You say you've had your desktop for over a week?
Throw that junk away, man, it's an antique
Your laptop is a month old? Well that's great
If you could use a nice, heavy paperweight
time spent in the box for electronics is money out of your pocket in most cases.
mojo also asked a very prudent question: what do you intend to DO with it? i keep hearing from this person or that person if they should buy a new computer with more jiggahertz because more is better, right? more querying results in finding that they use their computer for word and email. they do not need a 2500 dollar rig to do that though they seem to want it. on the other hand, if you want to play games but you try to go cheap and build something for 600-700 bucks, you may find it's useless for what you wanted to do and are out that money for no reason. tell us what you want to run on it. how long do you expect it to last? how much money do you have to spend, roughly, beyond "as little as possible"? by giving us a better idea of your needs, we can more effectively help you choose what you want.