Is this worth the $100 extra or can it be exploited only by certain software programs designed specifically for it?

Intel Hyperthreading (http://www.intel.com/technology/hyperthread/)

Yes, The software would have to be designed to make the most of either a a processor with HT, or a multiprocessor rig.

Thanks. I think I will just get the regular Pentium 4 when I finally order a new PC.

ahh shiet, I'll just spam the whole board.
Build your own.
http://www.newegg.com

newegg is great, but I think the current concensus is that the trade-off between building one and getting a built one is not much.

How about this question, is a Pentium 4 necessary compared to the lesser know competitiors?

I was looking at an add from Circuit City for an eMachine

Celeron D (2.53Ghz, 256KB L2 Cache, 533Mhz FSB)
256MB RAM
40GB HD
16" flat CRT

and one of the worthless printers that are thrown in most of these packages (they get you when you need more ink)

and after rebates it is $400.

Is there any history of these non-Pentium processors causing any compatiblity issues?