Core 2 Duo E6600 - $315 (per 1000 units. Final price might be $5-$10 higher).
2 Gigs DDR2-800 high-quality memory - $169.
SLI motherboard - Can only guess, but $130?
You could go for the lowest Conroe, 1.86Ghz E6300. That is better than a X2 4800+ at stock, and also overclocks to close to 3.0Ghz on stock air.