For the time being, the best advantages of both quad or dual core processors is seen in server operations, business, graphics and scientific endeavors, or when multiple high CPU usage programs are run at the same time. Otherwise, the operating system, other software and the end user are the primary limiting factors. Other than for "bragging rights" and ego soothing, the average home user will not realize or even recognize much of an advantage in using Quad over Dual processors any time in the near future.
For now, the Vista operating system decides when or whether another processor is actually needed to handle the load unless the user directly intercedes and assigns other processors to be used by specific programs. Unless a person is working on several projects at one time that requires a lot of CPU usage, the additional processors will be idle for the most part. Thus, the quad versus dual waste of money argument applies; at least at the moment.
Depending on the cpu's core clock speeds and the actual games being played, a single core processor may actually give somewhat better performance with some games than either a dual or quad processor system. Most current games are written for use in a single processor system. This is rapidly changing however. A few of the new generation games are taking advantage of or are planning on taking better advantage of multi-processor systems. Game code and other software authors will write more for dual and quad processors as those processors become more prevalent among average users. The new first-person-shooter games seem to be leading the trend where more photo-realistic, cinematic 3D displays are in greater demand.
Meanwhile, if money is not an issue, then go ahead and invest in the most awesome computer system and periphreals that you can afford. The code writers will eventually catch up. Investing in Quad may be overkill for now, but you will be ahead of the upgrade game that is played by the software companies and the hardware manufacturers. You are correct that Quad-core CPUs should still run your games.
Since the 32-bit version of Vista is limited to 4 GBs of physical memory of which only 3.12 GBs is actually available for use, 3 GBs should be adequate for awhile. Those limits are built into the 32-bit system architecture. You might consider a 64-bit system. With a 64-bit system that memory limit goes up to about 128 GBs. However, much as with a quad core systems, software remains something of a limiting factor to consider when switching to 64-bit. Therefore, get the best quality and fastest RAM that your system and wallet can handle.
With the GeForce 8800, choose either the "GT", "GTX" or "Ultra" versions for best game performance. Go with a Crossfire or SLI (depending on your system) dual video cards setup if money permits or consider the newer GeForce 9800 series.
For the Hard Disk Drive, consider including one or more of the Wester Digital Raptor drives (10,000 rpm versus 7,200 rpm) for speedier HDD access times.
Whatever you decide is best for you, happy computing.