I'm in Canada by the way...and i have my own computer supplier...so i get wholesale cost. (I own a tech business...just looking for second, third, fourth, etc opinions)
Yeah, the Celeron D does NOT perform as well as the Sempron. Even if it did, the Semprons are incredibly overclockable whereas they haven't had as good of luck with the Celerons.
Mundo i dont think u got goona get your whish man, u want to run nuendo with full vst libraries you goona need more then 1 gig, more toward 4 gig. Im petty sure amd 3000 or p4 3.0 ghz can handle the load. But the ram is goona kill you.
It's probably make that budget gonna go to about 500, but i would go for a socket 939 and get an Opteron 144 for about $170, and overclock it a bit, even with value ram. As for the motherboard, get I would recommend an ASUS A8N5X ($80-85) or just a generic motherboard that's compatible for Opterons.