I built mine a copuple months ago, and can be called semi-high end for $800 (already had the monitors).

Here's what you want, and while these specs are jsut a little better than mine, not building it yourself will about triple the price.

Core 2 Duo 2-3 ghz.
500gb-1tb HDD in a striped raid array.
2-4gb RAM
256mb+ V card. If your a hard core gamer, look at the 8800 cards (c $500 itself)
DVD burner drive. x2
Card reader.
5.1 (or more) sound card (built in or seperate card, about the same unless you're a fanatic)

If you need monitors, and are a hardcore multitabler, look at 2 20"-21" LCD screens.