I bought my current computer wholesale and put it together myself.
Here's what I bought:
AthlonXP 1800+ processor
256MB RAM
GeForce 3 TI 200 128MB
AOpen AthlonXP Motherboard
Case and 350W Power Supply
Bought that about a year ago, and it all came out to about $750 CAD. If you added a 19" monitor, 60GB HD, and DVD-ROM, it'd probably come to about $1100 CAD, which is only about $810 USD. Mind you, this was wholesale (i.e. straight from a distributor, no profit to be made by a store) so you may not be able to find quite the same deal, but if you can, you should consider doing that.