If I were you, I would use my money a different way...
Get the 8-core machine, and instead of the apple monitors, Get 2 dell 24" monitors. The savings right there will be enough to cover the cost of the 8 core machine.
Or just get the dell 30" widescreen monitor in the outlet for 1049. Thats a $750 savings which would be the cost to upgrade to the 8 core machine
I've SERIOUSLY been thinking about that. looking at going with a 24" gateway and keepint the 17" cheapo I have on my windows box. Get a calibrator, and don't touch the controls after its calibrated.
The Gateways are brighter, cheaper, faster response time, etc.
I think I may have just talked myself into it!