My last 3 computers were all under $700 and I just ordered a laptop for $1675.
Its cheaper to order a computer from cyberpowersystem.com than it is to customize it yourself and build it.
The most expensive types of computers are designer "boutique" systems like AlienWare, and even though its not a tiny botique computer Sony is also way overpriced. On a typical Sony you pay about ~25%+ extra than you would with another company for similar specs just because they put the name "Sony" on your computer. (That goes for all Sony products including TVs, Cameras, etc.)
The mid-range type of PC is I guess whatever you buy retail like HP, Compaq, Dell, etc.
I think any computer is great if you know how to keep it. That means not installing a bunch of crap just the essentials that you need to get your work done and maybe a few games. Definitley NO file sharing of any kind. For MP3's install Listen.com's Rhapsody service.
That along with an anti-virus program and Spyware Blaster regularly updated would be enough to keep a good configuration working for a long time without needing to format or having any problems.
But no matter what computer you've got I think the best way to get a good start on a good configuration is to install a fresh copy of Windows not the bundled version and keep track of what should be on your system. A clean computer is a fast computer.