I'd suggest going with an RDRAM solution instead of PC2700. RDRAM is still the best memory solution for the P4. Coincidentally, that will change the MB. Also, as was mentioned above, what exactly is your father going to need a maxed out P4 3.06 system for? Unless he's a hardcore gamer, does a bucketload of digital video editing, or regularly compiles massive applications and renders huge animations, he simply doesn't need that kind of power. Of course if he's the "gotta have the baddest boy on the block" type, then he's certainly getting it with this rig.
To answer another question, XP will indeed see multiple processors. It sees the HT technology as 2 logical processors, and is able to utilize them as if they were actually 2 physical devices.