Not much. Intel's 4 cores + hyperthreading no longer counts for bigadv, and it's only with bigadv that running Linux has a benefit - running normal SMP it's better to go with native Windows.
Technically, while 8-core systems still count for bigadv, because Stanford is not really assigning WUs for it any longer in the process of phasing it out completely, if you do run the -bigadv flag, you'll only get SMP units for the most part. They're actually also doing the same for 12-core bigadv; in the future the minimum requirement for bigadv will be a 16-core system, preferably 16 native cores - according to some in FF who have tried it out, 8 Intel cores + HT requires at least 2.7GHz to make the deadline for some of the projects.
This is also why I suggested Mark get dual 6128s with a dual-G34 motherboard (I know I was late, though) - the price would not be that much more than for a single 3930K, and it would last in bigadv much longer and much better.