Still, that might be a little tight. You're looking at about $100 or $200 for RAM (depending on 1gb or 2gb), about $100 for a HDD, $100-$150 for a case and PSU. If you want a CD or DVD-RW, you can get that for about $30. This is all assuming you don't want anything too fancy. That leaves you between $500 and $290 or so (with a DVD and CD-RW) for a processor, motherboard, and video card.
For SLI/Crossfire, you'll probably be spending at least $300, so assuming you get 1gb RAM, a cheaper case (you want a namebrand PSU at least), and no DVD or CD drives, you could get an X2 3800+ for about $120 and then a decent $90 motherboard all for within $800.
Like I said, that's kind of tight.