There are a range of options. If you're building it yourself, you could get a dual-core AMD-type motherboard, buy an inexpensive single-core processor, wait for dual-core prices to drop before they're no longer in production, then swap out the (e.g., Venice) for an X2.
Or, you could spring for a dual-core right away. You WILL notice a difference when running multiple applications simultaneously.
A point I made elsewhere and earlier was that 64-bit processing is still waiting for software to catch up with it, although I should probably check again to see what latest developments in that market have occurred. Otherwise, 64-bit only assists the OS in currency right now -- the 64-bit version of Win XP. And 64-bit processor technology is now well established, with Intel coming in as an afterthought with EM64T.
From what people say here, dual-core processors provide a real noticeable improvement for multi-tasking. It certainly beats Intel's HyperThreading technology, which has upsides and downsides with an improvement in speed of 10% and not much more. Dual-core processors provide you with 100% gains in a multi-tasked environment.
You're advised at this point to stick with the AMD dual-cores. Tom's Hardware Guide proved that this spring.