I don't know if you can choose components, but here are some tips what to look for:
The graphics card is the most important part, basically. The ATI 9800 Pro or GeForce 6600 GT are both quite good and not overly expensive (guessing about $250), for example. Don't go too low here, but you won't need the highest top end card, either (they cost easily twice as much).
Processor speed is not overly important, any processor you can buy these days will do fine, tho the mid-range ones are usually not much more expensive than the lowest ones. Just get one with a good cost-performance ratio (cost should be around $100~150 for a CPU with about 2~2.5 GHz). High end CPUs are a waste of money pretty much.
Motherboards are not so different as they were a while ago, tho I'd still stick to a manufacturer with a name, like ASUS. Can't do much wrong there, tho you should look out for what stuff comes onboard with the motherboard, they often have soundcards installed, for example, while they are not as good as seperate soundcards they usually do their job quite well (if you want surround sound, you'll need a seperate card, of course).
RAM is very important. 512 MB is a bare minimum, 1 GB is better.
Then just look for the accessories you need (like CD/DVD Burner, HardDisk size, etc).
So, a system with an average CPU, good RAM and a good graphics card is what you need.
Bye
Thanee