Approximate render time in After Effects CS6
Sadly this isn't really answerable in an exact way.  There is simply too many variables at play.  How much motion is in your source and final videos as well as how computationally and memory dependent your effects that you are applying will all impact the overall performance.  Your best bet for getting an idea of the performance increase is to look at what is currently resource constrained on your system.  Are you maxing CPU usage, memory usage, graphics card processing or disk access?  Increasing whatever you are constrained by will produce a significant improvement.

Based on your current configuration though I can try to take a guess, you are really underpowered in pretty much all categories other than disk access.  Dual core 2.4 ghz with 4gb of ram and a low end 512mb graphics card is nothing when it comes to rendering.  I'm actually a bit surprised it manages an hour in 6 hours.

To give you an idea of a comparison, my computer is a 2.93 ghz original i7 with 12gb of ram, striped SSDs and a Radeon 6970 (which sadly can't make use of the CUDA support).  I can comfortably render clips that are higher than 1080p quality with 2 pass VBR compression in just over twice the runtime.  With the CUDA support on the GTX, you'd probably see even more of an improvement.  For your 720p case, I make no promises, but it wouldn't surprise me if it would run faster than real time.  (Not to mention the fact your experience just using the software will be infinitely better.)