Well, as of now Intel is really pushing their new Penryn processors, with the first of the bunch already released (e8400 - 3ghz dual core). It is an incredibly fast processor, despite only being dual core.
Something to mention about deciding about quad, dual etc. Not a lot of software can support quad core and it all depends on what you plan on doing with it. Intel at the moment is ahead of the game and with these Penryn processors, they're really upping the ante.
I have built machines with both AMD and Intel processors and have had good results with both. It just seems that Intel is pulling ahead of the game and really putting out some solid processors.
By the way, I'm an Apple guy now but I still keep up my research on the debate between AMD and Intel.
Good luck,
Kegan