You could get a workstation, both Dell and HP are pretty much leaders in this area, but unless you have a specific use for a workstation it would not be overkill, it would just be a complete waste of money. A workstation can run anywhere from $2500 on the very low end to up to $10,000 depending on what you want. I was playing around on Dell's site and for a dual Xeon 6 core chip and 24GB of ram you'd be looking at about $9500. You have to remember these workstations have specific uses, if you just want an ultimate gaming system this is not for you. I'd imagine you'd run into driver incompatibilities that would either prevent some games from running, or at the very least make the experience far from ideal.
Those 6 core chips don't even perform that well, if you want over kill get Intel's 6 core chip. At $1000 it'll be a lot cheaper than 2 Xeon chips and you will be able to buy a motherboard geared for gaming and overclocking. Or better yet, wait, because right now Intel just released their new chip architecture. Later this year or possibly early next they will release their new high end chips and by that time AMD will have released their new chip architecture as well.
This really isn't a good time to be building "the best of the best", because the current high end chips are about to be discontinued. I'd suggest waiting for Intels socket 2011 chips as well as AMDs Bulldozer chips that should be out this summer or fall.
