Ok, here goes the power of the ZEN: Ready?
Buy an ASUS or GigaByte SLi capable nForce4 chipset MOBO if you can afford it, but they are much cheaper now.
I agree, buy an Athlon64 3500+ processor, or a 3800+, or 4000+, depending on your budget. If you have a HUGE Budget, go with the 4000+, if it is a smaller budget, go 3500+ , etc.
Next, buy two nVidia 6600GT PCI Express Cards if you are on a tight budget, or two nVidia 6800GTs/Ultras if you have a larger budget. The 6800s are much more powerful, but the 6600GTs are still kick ass!!
Ok, to overclock, you need a few things to know. Firstly, do you know what the BIOS is? Secondly, there is a multiplier on the Motherboard (small switch with tiny levers (switches) you change to change voltage settings and FSB mulitiplier. Then use the BIOS multiplier to change the total processor multiplier, and Overclock the processor. EASY. Do a google search for more details on how to overclock a CPU.
HOPE THIS EXPLANATION HELPED SOME. GOOD LUCK!!