You can build a HTPC in the $600 range, but most likely it won't fit your requirments for performance. If your not computer savy, there are lots of online etailers that can provide decent systems starting around $1,000 and higher.
If you google "HTPC" or "media center pc" or "home theater computer" etc.... you will find lots of hits.
http://www.ecollegepc.com is an example of a company that makes good Media Center pc's at had to match prices that are customizable.
Most likely I'll build my own, but I've built pc's for years so it is pretty easy for me. HP makes some decent media center pc's now, but are a bit pricey.
