Just wanted to say that my Phenom quad-core overclocked to 3.2GHz seems to run morrowind fast even with a buttload of script-heavy mods, so don't necessarily avoid higher core CPUs unless the price is too far out of your range. Morrowind may not take advantage of more than a single core, but more and more other progams and even games will start to. It may not be possible to "future proof" a computer system with how quickly things develop, but that doesn't mean you should ignore it completely. I am assuming you'll want to do plenty of other things aside from playing Morrowind on your computer. And, btw, you don't need a >$200 card. A GeForce 9800 GTX+ can run it even with very high settings in MGE/ENB and can be obtained for less. I've heard bad things when using ATi with Vista, so I'd avoid ATi if you are not using XP.
Actually, if you use other programs besides morrowind, such as ENB and MGE, your other cores will work for those programs, allowing you a much higher potential then a single core setup, and even a duel core(in some cases) when it comes to Morrowind. Before I had MGE, my core usage was somewhere between 80% on one core and 5% usage of the second core, to now utilizing close to 60% of the second core, and not suffering any loss in fps. I highly recommend looking for multicore optimizers designed specifically for your CPU, it is amazing how much better my 6000x2 runs since installing the optimizer for it.
Also, while I agree that intell does have an edge over AMD, the edge is narrowing down to efficiency more then speed, especially when looking at AMD's newest black edition offerings. They are playing catchup, but its a race of inches, not miles. They are close enough now, that a 6400x2 with its very moderate pricing, is probably the best bang for your buck when it comes to cpu's to run morrowind with. Semantics aside, I prefer the pricing and upgrade structure found in AMD's am2(soon to be am3) line, but Intell has some very impressive tech in its I7 line which is more efficient and faster then the best AMD has to offer, but it still comes down to inches. If money is an issue, AMD will more then suffice, if money is not an issue, I would probably go Intell for a new build.