(Sorry for the delay in responding, I've been working 50+ hour weeks for the past couple months...)
Before I make any recommendations, I'd highly advise waiting for a month or two if you can. Both Intel and AMD are making some major changes to their CPUs. None of the current motherboards make for either the Athlon 64 or the Pentium 4 have any newer CPUs planned for their corresponding sockets. (I.E. There is no CPU upgrade path for any existing motherboards.) There's a lot of other reasons to wait, but unfortunately I don't have time to list them all at the moment.
If you really want to buy something now, and you want a Pentium 4, I'd have to recommend using the Asus P4P800 line of motherboards. (The P4P800 Deluxe is excellent.)
For the Pentium 4 itself, make sure you go with a 'C' type Pentium 4 (Northwood type). Avoid the newer 'E' type Pentium 4s (Prescott core). Even though they have a 1 MB cache, they are actually slower overall and run MUCH hotter than the Northwood type Pentium 4s. (I have some doubts about their long term reliability given the amount of heat they produce as well.) For memory, you really have no choice other than 2 sticks of DDR400 memory. (2 x 512 MB sticks = 1 GB of ram for example.)
On video cards, the ATI Radeon 9800 Pro is a very good bang for the buck card at the moment. It's worth mentioning that the new Radeon X800 cards are now available, and 2x as fast as the 9800 XT, but they also cost at least $150 more than you can get a 9800 Pro for these days.
