If that's all you're doing with the PC, you can probably use whichever is cheapest.
HT or dual core are the only tangible benefits you'll see. Differences in clock speed are pretty negligible so go for a good bang for buck price point like 3200+ or 3.2GHz and go for it. I think in your case you will recognize the benefits of HT in a P4 more than the A64 advantages, which are primarily in games.
The P4 has the obvious power consumption downside. The A64 will be cheaper in the short AND long term, but you give up the slight multitasking advantage of HT.
With your usage profile I wouldn't worry too much about the decision. If you're more comfortable with Intel, then do that. Don't let people talk you into a processor you don't really want. Realistically, they aren't that different. If you're overclocking the A64 is probably a better platform for that, but personally I don't really 'feel' a 600 MHz improvement when I overclock my A64. I suppose it makes single apps faster, and the task switching is as slow as ever. Of course certain games show improvement, but my games are, for the most part, not CPU bound at the resolutions I play at.
My opinion is you should be deciding whether or not you need HT, as that's the only reason to go Intel at this point... it's the only thing in my experience that actually gives a 'feeling' that the machine is faster. Increasing clock speed is good, but honestly, I really don't notice it that much doing just regular stuff.