From my G+ Feed:
I was totally not completely wrong to buy a mid 2010 Mac pro!
...because of course right after I bought it, it was discontinued. And I bought it for teh upgradez!
Anyway, I just:
1. Upped its RAM amount to 32GB (Apple begrudgingly updated their web site to finally admit that, yes, 8GB sticks are supported) (http://www.frys.com/product/8008624?site=sr:SEARCH:MAIN_RSLT_PG)
2. Switched to a 980GB Patriot Ignite SSD (http://frys.com/product/8358357?site=sr:SEARCH:MAIN_RSLT_PG)
3. Swapped the video card for my "old" PC's GTX660 -- not officially supported but totally works except EFI boot. Old but still 2.5x the performance provided by the original HD5770, plus access to Cuda.(http://www.frys.com/product/7551666)
In summary: I am getting the same level of upgradability as I would from a hackintosh, for what ends up being a similar amortized cost, minus the headaches.