A budget HTPC can be made for around 400-500 USD without monitor.

Just be sure that you buy a HDPC compliant video card and monitor.

If you plan to put the monitor on your desk, I think 24-32" LCD is pretty good size but it will cost you. DVI and/or HDMI inputs are fine since you are able to connect from your video card with a cheap DVI to HDMI adapter and most video card include the adapter.

If you plan to put the monitor in your living room with the HTPC besides your home theater (or from another room), I would suggest 42" or higher LCD HDTV. Plasma gives you better picture but it's not very good for static images from your PC.

You can post the parts you plan to buy so our knowledgable members can give comments.