Hey! I love dell servers. Lots of clients using them. Stupid expensive to replace what should be "cheap" parts (ie: purchasing a special power supply for a Dell is not cheap), but the servers themselves are very nice.
(my 2 cents, but we don't SELL Dell, we build ours ourselves! LOL)
Remember that when you add power to a single server, no matter how you do it ... it's still a single server. We've found that 4 cores is the breakpoint for power vs ROI. Some say it's the 8 core (dual 4 core) Xeons.
But in this regard, I have to bow to Matt, who has significantly more experience than anyone else with multi-server setups (not just a single office experience, or even a couple or a few, but LOTS of offices with multi-server).