There are actually quite a few recent and upcoming titles that are designed to take advantage of multithreading, with the Alan Wake devs going as far as saying that to try and play their game on a single core is just foolish although possible with hyperthreading (albeit at a huge performance penalty).
Tom's Hardware has done a CPU shootout, and one of the games they have tested that is designed to take advantage of multi-core CPUs is Supreme Commander (which is getting a HUGE expansion soon btw).
Surprisingly though, the dual core 6850 still scored higher. However, games like Alan Wake and Crysis may give the performance advantage to the quad-core, not counting all the 2008 titles (unless you're planning to upgrade your computer every 12 months). It sounds like you really can't go wrong with either though, with each having little advantages here and there, and they are the same price.
I bet the q6600 would score higher if it was matched to the e6850 3ghz speed.