quads increase the processing performance in applications that are built for them specifically. however, the difference between a single core and a dual core in any application is absolutely enormous - the difference for a quad is negligible.
there are a lot of multi-threading-capable applications that say that they're good with quads, but that's just marketing crap. in reality, the quad is still shits and giggles until the majority of the programming market is optimized for it [note - THIS INCLUDES FL STUDIO, REASON, AND MOST OTHER MAJOR DAWS AVAILABLE]. hell, vista isn't even optimized for it, and there still hasn't been word as to whether w7 is. even some of the top games out aren't really built for more than a dual-core, and some not even that - far cry 2 was the first game i ever played that accessed more than one core for processing gameplay.
edit: i missed the multi-core thing phill mentioned. there's a reason we haven't seen anything more than that 6-core server cpu that intel put out at the end of last year. based on extreme multi-core tests, more than 8 might actually *decrease* the performance over a system with 4, 6, or 8 due to the simple fact that they don't have buses capable of handling that kind of bandwidth. moore's law in its virginal form applies here (i think it's moore's law, i don't remember off the top of my head), too - memory simply isn't expanding fast enough for cpu demands.