I still have to say that even after all this time (almost 1 year after originally posting the question) it still isn't really beneficial to have a quad-core over a fast dual-core. And it'll remain the same until software is rewritten to take advantage of multiple processor cores.
However, I must say that the Intel Core i7 (Quad Core) is really nice in the fact that it is just plain fast. Especially with the memory controller on the CPU instead of somewhere else on the motherboard; this allows for things to run much faster and actually causes the hard drive to be the only bottleneck of the system. But even this cpu isn't fully utilized without proper multi-core software.
It still seems (unless you are doing multi-core programming) it may be better to stick with a faster Dual Core, rather than getting a Quad Core. What I mean "Faster" is a Dual Core that has a faster clock speed per core than the Quad Core. However, the Intel Core i7 is a different architecture than the Core 2, so even the slowest Core i7 (the 920 at 2.6Ghz) is still faster than most of the Core 2 Duo's out there.
If you don't want to buy/build/rebuild a new PC, then one of the biggest things you could probably do today to improve performance with your Dual Core (more than getting a Quad Core) is to get a Solid State Drive to replace your Hard Drive. In almost every system the Hard Drive is the biggest performance bottleneck, and this will help it catch up to the CPU and Ram quite a bit. However, again, the Solid State Drives are still fairly expensive.
In the end, if you have a decent Dual Core that suits your needs, I'd say keep it until a newer multi-core (Quad Core or larger) is more fully utilized by the software you use everyday. Especially since Quad Cores are available Today, buy in the next couple years we are likely to see 8 Cores, 16 Core or and eventually more, and maybe someday ALL of our software will more fully utilize them (I'm looking at you Visual Studio, Firefox and MS Office).