I run a quad core with 12gb of ram with reason 6 without the 6.5 update. I use mostly combinators and both music and effects. Reason 6 is not Reason 4 as far as cpu efficiency. It can suck up some juice depending on your arrangements. Those extensions can also do the same from what people tell me, which is another reason I am sticking with 6.02.
Best to upgrade to a quad core 64bit machine, and make sure your using a very capable sound device. I had to upgrade from a audigy 2 card to a mbox 2 (the one with midi) just this week and it really makes a difference. I was getting audio stuttering and CPU warning with my quad only at 60%. If the audio device can't handle the data then you will get those messages even if your cpu is humming along.