well, if you're lookin for a good machine for under $1000 (USD, i'm assuming), then i'd recommend checking out eMachines computers in Best Buy or Circuit City.
they use a mix of off-brand parts and name brand parts, but they've proven to be quite reliable with good tech support, and they're cheap.
things to look for:
Processor- AMD Athlon 2800+ or better
Memory- 512MB DDR-333 or better
Graphics- an AGP upgrade slot
if neccessary, ask the salesman to bring up technical specs and ask them to tell you what kind of upgrade slots are open on the particular model you're looking at. trust me, the AGP slot is vital
to you having a good machine. and the reason is simple: eMachines does not include video cards with their computers. you'll have to buy one seperately.
for the video card, i recommend either an nVidia GeforceFX 5200 or an ATI Radeon 9200. if you can fit better cards than those in your budget, then go ahead and get them. those are just the cheapest cards available that will run most games without many problems.
now, things like a big hard drive, a dvd-burner, and other little things should be considered if you're going to use them
. otherwise, its just wasted cash.
now, i know this is a lot information, but you have to consider a lot of information before you buy a machine. just keep these things in mind, and you can easily get a good machine for under $1000.