I have a similar board (ASRock 775i65G Rev. 2) with a Pentium 4 531 (3.0GHz, 1MB L2, HT), with 1GB RAM (Kingston CAS 3 DDR 400 - Not certified to overclock to 1066MHz by ASRock) and I am wondering if purchasing the new Pentium Dual-Core E2140 (1.6GHz 1MB Cache) would be an upgrade in terms of xvid encoding, file serving, and gaming at 1024x768 (Windows XP). I have an Powercolor ATi 9800XT video card and for the games I play (WoW, Counter-Strike, Neverwinter Nights) it's perfectly fine, but when I am not gaming it still heats the house quite well...
The idea here is that I really don't play games much anymore (and when I do, I generally play from my laptop anyway), so my desktop is acting more like a server these days. Infact, I have been considering popping out the video card to save some electricity. My powersupply has seen better days and I'll be replacing it with at a high effeciency model shortly as well. So, with the E2140 being so inexpensive and power efficient, it seems to be the best bet for an upgrade - provided it will out perform the CPU I currently have.
As of yet, I haven't found a Pentium 4 HT vs Pentium D vs Pentium Dual-Core review, but that is essentially what I'd like to have look at.