Pragmatically, what you're doing is attempting to guess a large numeric value, and your computer is issuing a whole lot of guesses to try to find that answer. The whole guessing process—what's called "mining"—is entirely bound by the speed of your processor, or the speed of your co-processor when you have additional computing hardware present. At how many guesses your processor can make. The faster your processors and the more processors you have, the more guesses your configuration can attempt.
Dedicated mining systems are far faster at this guessing than any rigs based on Intel Xeon or Core processors, too. Far, far, far faster.
Xserve is far too old and far too slow and far too power-hungry to make an effective mining rig. And it's loud.
There's no combining of mining rigs. Not in the fashion you're envisioning. You'll have two or more (loud, old, slow) servers in a rack, and the rack will get quite hot as the Xserve servers get very hot under heavy processor loads.
To get Xserve to mine faster, you're probably headed toward adding a GPU and finding mining software that runs on it, and Xserve supports very few PCIe or PCI-X slots. GPUs will typically mine—guess—faster than Xeon or Core processors.
Run some numbers and see you're recouping your electricity costs here, too. Unless you're also using your mining rig as some fairly expensive residential heat, of course. There are various profitability calculators around, and that'll give you some idea of the costs and the performance that modern purpose-dedicated mining rigs are obtaining.
AARP has a good quote here, too:
“Bitcoin — A bunch of computer code that a bunch of criminals, idealists and speculators agree is worth ‘real’ money. Sadly, its real-money value swings widely, making it impractical except for criminals, idealists and speculators.”
That's certainly not particularly charitable to folks that are mining or otherwise participating, but the volatility is an ongoing factor for all of the folks involved in the cryptocurrencies, as is covering the costs involved with the mining; the costs of powering and cooling all that guessing. And remember about covering your costs from the occasional fraud or other untoward event, when some exchange might issue everybody a "haircut" or when some exchange is breached. Your hashing/mining/guessing rates have to cover all of that.
