You can get a decent box for under $500 USD.
Here is a store I deal with. I usually start with the base model then tell them to put better components in such as a HD power supply, more ram, bigger HD, additional fan, better DVD burner etc. Their base $400 CDN system works out to $550 CDN after that. Throw in a 15" LCD panel display and you are looking at about $780 CDN.
Note that for higher end video processing, you may want to get a dual board, but it's still a good price.
Note: I always get the systems without any software.