-2x Intel Low Voltage Xeons
-8GB ECC DDR2
The 5000x or 5400 would have been a more faithful replication of a real MacPro the power drain was just too big. FB-DIMMS suck power, the north bridge sucks power. For a machine I have running at least 8 hours a day it's a factor worth considering. The TDP on these Xeons is actually lower, combined, than the TDP of my current Quad core chip. So that's 8 cores for the (energy) price of 4.
Also in terms of cost, I didn't really want to go with the 5000x as its an old chipset and may not have support too far into the future and you're still talking $200 or so for the board The 5400 is more modern but then you're talking $350. Plus the added cost of FB-DIMMS for both. The 5100 was $300 but I'm saving $75 on RAM right off the bat and when prices drop a bit more I can go for a full 12GB if I need it. (I don't see how I could but why not?)
Performance-wise the memory bandwidth is quite a bit lower. Probably half. but when you're talking 10GB/sec vs. 20GB/sec I'm not sue it truly matters. Most of my needs -- audio -- is going to be lower bandwidth but more latency critical and that's an area DDR2 has FB-DDR beat on.
OSX compatibility should be good. A couple people on the InsanelyMac forums have things working more or less. I'm a little concerned about the changes in 10.6 but hopefully the X-Serve platform will move to 5100 and it won't be an issue. If not, at least the south bridge is ICH9R so half the battle is already won. As far as I know, the OS doesn't usually have to muck around with the north bridge a lot. It should just work.
Overall I think it'll be under $500 but not by much. Good excuse to work some more OT next week I suppose. The Global Knowledge people gave me a $250 gift card plus $50 in PayPal coupons, plus the 20-30% off live.com is doing. Not bad for 8 cores. Plus I can sell my Quad Core setup for at least $200. I should end up making money out of this deal.