Ok.. First thing's first..
32-bit XP and Vista will not support more than 3.25GB of memory.. You can physically have 8GB installed, but the OS will only see/use 3.25GB max.. There is no limitation that *I* am aware of on the usage one single application can have of that amount..
Concerning dual-core and quad-core CPUs, even if your preferred application does not specifically make use of multiple cores, your performance will still benefit from having multiple cores.. You can, if need be, manually set which core of the CPU an application is running on.. For older programs and games, this is sometimes necessary.. You move all applications to the first core, and put the program you want to run on the 2nd core by itself.. This will improve performance greatly in programs that don't directly support multi-core technology..
As for your single 3700+ vs. one core from the 6000+, the 6000+ will blow the 3700+ out of the water..
The cache on the CPU itself is a huge boost on the 6000+, and simply put, the technological advances from the 3700+ to the 6000+ are huge..