Hi ericvpi, I'm glad you posted this board!! I'm with the same dilemea as you.
I want and need a new computer, but I'm not sure to buy one now, or be patient and wait until early 2008 for Penryns to be released. I've thought long and heard and tried to figure out the solution, so maybe you and I can find the answer together.
Looking at the specs, a Quad 6850 barely beats a Duo in any game. When you see multithreaded applications, the Quad pulls ahead by a nice little cushion. Now, I'm a gamer for sure, but I tend to play my old favorites a lot, besides some of the newest releases, and something like Crysis is quite intimidating looking. And like you, I have been stuck between the 3.0GHZ Duo or the 2.4 Quad, and for me, it's a price issue; it sounds like it may be the same for you! It's a very, very tough choice to make, especially since the 2.4Quad is a lower bus then the duo (1333 to 1066 respectively).
So, there are two good options I believe feasible for you, my long-sought friend: Either purchase a Duo and stick it out gaming and hence forth for as long as you can (which may be quite a long while!) and purchase a nice new Quad Penryn when prices drop in the future when you need one, which may not be to 2009 (if you're not too disgusted with some supercharged Nephram processor or such). Or, you can purchase the 2.4GHZ Quad, and be safe-guarded against the full use of SLI graphics, and if you're like me, like to have multiple applications running while I play a game on occasion, or want a computer for the purposes of longevity (4 years+) and want to make full use of those future games that are quad-expectable.
So if you don't want to have to waste a perfectly good Duo in the future of upgrading, and want to squeeze the longest life out of your upcoming computer, I've dissected it's probably best to go with a Quad 2.4, even with the lower bus, and lower core speed.
I hope I've helped. Good luck my friend, let me know what you decide!!