dual processors are the way to go. every now and then when im tracking vocals and i get that vocalist who needs 20 to 30 takes for one verse. while im waiting for them to get sh*t together,i like the luxury of being able to browse the net, play a game, or burn a cd with out disrupting any of my open audio applications. dual processors really doesn't make anything run faster, i just gives you more power to run more things. your 2100 would be your corvette. your 1800 dual would be your expedition. the number of passengers would be your tracks or applications.
u could just get the dual motherboard with one "2000" and install another one at a later date when get more funds. i seen a combo for $430 at pricewatch.
just my two cents.