Just let him win. Get a Pentium D 820, or if it's an option (price) that nice shiny new X2 3800+. If you're not gaming, though, the Pentium D will be an excellent multitasker.
However, if you're set on AMD, here's a basic rundown of AMD's advantages (in the most basic terms I can put them into)
AMD CPUs...
- run cooler and use less power
- perform almost all tasks faster than Intel's offerings (except in multitasking situations, as already pointed out)
- access memory faster
- Can process 64 bit code along with 32 bit code and the like (some higher end Intel CPUs can do this as well)
- all recent generation parts use the same motherboards (except mobile CPUs, but they matter little), so they're a better upgrade path (though AMD IS changing sockets next year, they won't phase out Socket 939 for some time yet.) Intel uses different motherboards and chipsets for single and dual cores, so if you buy a single core now and want a dual core later you have to buy a new motherboard as well.
This isn't to say Intel has nothing going for it. Their dual cores are much more attractively priced (though AMD's new X2 3800+ helps to negate that advantage, it's still quite a bit more expensive than a Pentium D 820) and their mobile CPUs completely own AMD's in terms of performance and thermal properties.
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