Drew,
Have you really assessed the need for massive amounts of RAM? I'd urge anyone in your possition to do some homework first and try to estimate their RAM needs before making any decision. I am one who thinks the future is more on the disk streaming (SSD) improvements to be able to handle more, and not so much in having massive RAM capacities. It is simply more cost effective, to begin with.
On the other hand, just respectfully proving your assumptions:
a) An i7 3930k CPU and suitable x79 MOBO, with 64MB RAM go today for about US$1,000.
b) A six-core Xeon E5645 (US$ ) is an older gen CPU. You can certainly use it, however I'd have thought you'd be more interested in investing in a newer Sandy Bridge socket-2011 reincarnation, such as (for instance) a six-core 2.0 GHz E5-2620. This last would be around $425, plus a suitable double CPU MOBO, and 48GB RAM, would be give or take $1250. Then you may want to consider adding the second CPU, or not. But whatever you buy for the first, the second has to be exactly the same.
Either alternative is viable, I think it may be start to be more of a cost consideration at some point. The i7 system is a top line system by today's standards. The dual Xeon is a premium system, higher cost. Keep in mind there are a few things to keep in mind when you use these dual CPU MOBO's, in terms of how RAM is split, same CPU's, etc. And the fact that the E5-2620 is considerably slower than a i7-3930. When you go and look at a faster Xeon, the price creeps up very rapidly...
To me, bottom line, you need to estimate what your intended capacity requirements are for the next 3-4 years and decide. I'd not look for any technology decision you make today to try to overcome the pace of rapid progress and go beyond that timeframe, as you'll find yourself interested in upgrading in 3-4 years. I guarantee it.
Hope this helps somewhat.