My first point of advice is: read, read, then read some more.
The worst thing you can do is jump in without knowing how deep the water is.
SMP machines are somewhat different from single-CPU machines and a good deal more complicated, so read all of the stickies in this section and browse around a LOT.
2CPU.com has some information, although it is not particularly friendly to the SMP newcomer.
Now, you want an SMP system. How much are you willing to spend? That's what it really comes down to. For a dual AMD system, you're probably looking at $300 for the motherboard and CPU's alone. After that, you'll need a strong power supply (preferably 500 watts or more,) a case with good airflow, 2 good heatsinks, a SMP-capable operating system (Win2K, WinXP Pro (not Home Edition), or Linux), and all the rest of the usual computer hardware. A dual Intel P3 system will cost about the same. Dual P3 motherboards are cheaper than dual AMD motherboards, but the actual P3 CPU's are so expensive, they offset the lower motherboard cost. In the end, a dual P3 system is equivalent in price to a dual AMD system. A dual AMD system, however, will FAR outperform a dual P3 system, however, and have more upgrade potential than P3's. The P4 dual chip is the Xeon and they are REALLY expensive and the motherboards are REALLY expensive. Don't even think about that, unless you can lay out $600-1000 just for the CPU's and motherboard.
For the budget-minded, a dual P2 or dual Celeron system, is your cheapest entrance into SMP. Celeron's are only dual-capable up to the 533 MHz chips, however.
You can find cheap dual Celeron and dual P2 motherboards and complete systems on Ebay, but know what you're looking for, before you bid on anything. A lot of SMP'ers start with Ebay. Occasionally, you can find a really good deal on an old server system. Alternatively, you could shop for the specific parts you would like (motherboards and CPU's) and put the system together, yourself. This is far more fun and educating. A good start may be to look for dual motherboards that support P2/P3/Celerons. Buy a board and a pair of old Celerons and you've got your starter system. Get a little more money, later, and you can upgrade to P3's.
As far as bang-for-the-buck, most people would agree that a dual AMD system is a very good value and very good performance, but they are a little expensive on the front end, although not nearly as bad as a dual P4 Xeon system.
Whatever route you take, good luck. If you have any more questions, don't hesitate to ask, and WELCOME TO SMP!
