Well, here is what I can tell you for my experiences about upgrading and whatnot. Basically, I feel that it is a great time to upgrade right now because of many reasons. It's a great time to upgrade for YOU because manufacturers are doing it themselves, which means quality products starts selling for great prices. Here are a few reasons why I think you should upgrade:
1. A dual-core is plenty fast for most users, in fact its almost overkill for most users. Dual cores are becoming cheaper by the second because of quad-cores and just recently the new i7 quad cores. This effectively makes Dual-cores, which are already overkill for most consumers, a 3rd generation product which you means you get GREAT price/performance when you buy them now. (Compare the E8500 to the Q6600 from Intel. Unless you are doing Multi-tasking to the extreme, you won't need a quad-core and probably wouldn't even notice the difference between the two. So same price for better performance for MOST people with the dual core.)
2. Memory is also making a change, from DDR2 to DDR3. DDR2 you can get for VERY cheap right now with 4 gigs(DDR2) running into the 50 - 60 dollar ranges. 4 gigs, some would say, is almost overkill for most consumers, yet you can get it for cheap prices and thus providing another excellent cost/performance when you buy the memory now.
3. Upgrading to the latest and greatest is HARDLY EVER worth it. In fact, I can almost say that its never worth it. If you do a price/performance calculation when products just come out, they are HORRIBLE compared 2nd/3rd generation models. The fact that you are worried about spending a lot of money is already telling me that you don't need a "latest and greatest" model of computer and most people who think they do, really don't. (For instance, a gaming computer that wants to run Crysis needs the latest and greatest, but any other game doesn't need anywhere close to the specs Crysis needs.)
4. Greatest thing about the times right now, is that all of the companies (good quality and bad quality) need to get rid of their existing product to make way for the new models. Which means you can find Good quality products for great prices with great performance and dependability.
I don't mean to ultimately sway you in your decision, but if you needed a comprehensive comparison of upgrading to the parts of now, hopefully this helped.
Whatever your decision I wish you the best of luck,