Well, this is a very subjective thread you make. Reason is, it REALLY depends upon what you want to do.
If you want to multitask like no other, then the Intel
Pentium D is what you want. For some reason it handles more application better, however, when I mean a lot of applications, I mean, A LOT of applications. If you want to do some gaming, then this is where AMD
If you want to have better processing in gaming, then get the AMD, in most benchmarks, which you can find at http://www.tomshardware.com
the Athlon 64 X2 series processors generally take the cake. Although Intel does win sometimes, most of the time it's AMD. If you asked me, I'd say when it comes to sheer processing, AMD has it made. Intel is lacking in the memory controller area. Not to mention, their 'dual-cores' aren't techinically dual-cores at all. They're rather 'double-cores'. Because the two CPUs don't actually communicate with each other, instead, they communicate with each other via a Quad-Pumped front-side-bus.
This creates a bottleneck, well, a rather LARGE bottleneck. You see, the thing that makes AMD better in processing is it's ability to communicate with each CPU better, because they actually 'talk' to each other. Plus, they have a built in memory controller, which allows the CPU to communicate to the memory directly, as oppossed to Intel processors which I believe use the FSB once again.
If I was you, I would pick any Athlon 64 X2 series processor. However, if you're on a budget and you still want performance nearly identical to AMD, then get a Pentium D 9xx processor. Which ever you choose you'll be great;y happy with. Dual-Core processors are insanely fast, however, you don't need dual-cores just yet. Dual-Cores may seem like a much better option, however, they aren't full utilized just yet. Not to mention, Single Cores can compete with them extremly well in gaming applications. So, the choice is really preference.