I'd keep the P4 or upgrade to a Prescott, or best of all, get a 2Mb Cache P4 and go Windows XP 64-bit. Those programs are strongly biased to HT processors and are written to take advantage of memory and processor bandwidth so you should see a significant benefit in the bigger cache and 800MHz FSB of the newer P4's. A RAM upgrade in your current system would also give a noticeable increase for very little money these days.
This is an AMD centric forum and the previous poster even said
"unless your doing specialist tasks such as massive calculations, pushing network wide updates for a few hundred pc`s or encoding masses of video"
Two of which you are doing, yet he's recommending a S939 CPU? Where software is specifically written to push instructions at a processor (and Pegasus was modified for multi-threaded Pentium 4's in 2002 remember!) you are better off with that processor.
If you want to keep the AGP card, and I would argue that if you are replacing the processor and motherboard then why buy old technology so you can keep your old video card, then I would be looking at the DFI Lanparty 875 P-T which has Gigabit LAN, 8x VGA, takes DDR RAM and has compatibility with the 6xx series P4's. It's officially delisted, but if you look on Froogle for "DFI Lanparty 875 P-T" you can still find some.
If it were me, I'd buy a P4 640, an Abit AL-8 and a new PCI-E video card and RAM. That will keep you in headroom (1066 FSB) for a while and you're not investing in dead technology.
Please also remember that Northwoods have silly-money status on auction websites because they are perceived to run cooler and therefore better than Prescotts. A 3.2GHz Northwood P4 at auction will probably get £100-£120 that you can invest back into your 6xx series. Likewise, 9800XT's in good condition go for £70-£100! and a 6800GS or x800 GTO2 are only £130 these days and they'll just blast a 9800 (even the XT) for speed.