I'd ask in the Home Built forums about this. You're free to spend whatever you wish, but depending on your screen resolution, $1,500 is way overkill. You can probably get by with an $800 machine. Check out the $550 SBM to see how games did on that type of budget.
On topic of this forum: Based on how game companies are trying to keep 'platform-parity' with their titles, it almost begs the question about how much hardware you really need for new titles. Seems like a for a new system, a $120 CPU with 6850 or 6870 will give you ample performance to handle the type of 'least common denominator' titles that game developers seem to be moving towards.