Some online retailers such as Novatech offer bundles which include motherboard, CPU and memory - this will provide a base for your system.
Preference should be for the Intel Dual Core CPUs at present, although AMD offerings are generally a little cheaper.
Another method is to buy a barebones system. Some sources:
click here=*New*+Bigpockets+Top+Performance+4600%2B+Dual-Core+2.4Ghz+PC+System (new)
You should be able to acquire what is required within your budget.