Yeah, I upgraded my 2006 Mac Pro to 2 quad core 5355s (2.66) with multi-core apps in mind (Snow Leopard, then everything that comes after). Right now, the only thing that really takes advantage of multi-core is Handbrake, so if you do that a lot, then 8 cores is the way to go.
Frankly, the new Pros will smoke my 2006 octo-core, however, I think Snow Leopard will give the old girl a new lease on life. I think that will really be the way that apps are written in the future, so if it were me, the more cores, the better.
Just my $0.02,
JP