"I'm still not clear on the Opterons. Are they strictly for server side applications or can the average desktop/gamer make use of them for general gaming, etc? Don't the games have to specifically support dual processors?"
While Opteron based systems aren't strictly for server applications, they are generally targeted at that market. My new dev box is a dual opteron, and it's very well behaved, though when running 32-bit applications in general it's no faster than a dual 3 Ghz xeon, particularly cpu intensive applications. Tasks that are more disk/network/memory access bound, the system's server orientation starts to shine. This box compiles like a bat out of hell for example.
For consumer level use I don't recommend the Opteron, even a single cpu system. While 32-bit performance is very strong (it kicks the crap out of the Itaniums), they are really intended for 64-bit applications. 32-bit performance is only important for people that need to use a mixture of 32-bit and 64-bit apps. There is currently very little 64-bit consumer level software out there, and the only game I've even heard of potentially having a 64-bit version is halflife 2.
If you run engineering / cad / etc. type software, and there are 64-bit versions available, then going with the Opteron makes sense, since you can finally bust through the 2 Gb memory limit and 64-bit optimized code can run up to 4 times faster than its 32-bit counterpart. If you just plan on running typical 32-bit apps and games, buying a 64-bit cpu is basically a waste of money.
As to dual cpu's...yes software has to be designed to take advantage of more than one proc, though even non-multithreaded software can benefit as the OS itself can run more efficiently on a dual. You can also have one application processing in the background while working in another app if both are not multithreaded. If you do a lot of multitasking and any of your applications are cpu intensive, then having a dual can make life much less painful. Again though, if you don't have a clear need for it, it's a waste of money.