I had the same frustration with my iMac, although it was one that was relatively fixable.
The newest Macs have Thunderbolt, and with that you have certain upgrade options that are realistic since now there isn't such a penalty for using an external. Pretty fast even with a SSD.
There is, of course, the Mac Pro. Lots of upgrades for the older ones, but some of their technology is really dated. The new ones are almost too far in the other direction: if you just need a lot of hard drives inside it's overkill.
I think the best answer, and the one I adopted, was a hackintosh. Affordable, expandable, customizable. And easy on the wallet. Wound up using my funky iMac as a 27" monitor with it in TDM. Very happy.