With Vista 64-bit you will find a significant performance improvement, particularly in games if you have graphics cards with large memories.
It is easy to forget that in order for your CPU to address your video card memory, that video card memory has to be copied to your RAM first. So, anyone running a pair of 768Mb 8800GTX's in SLi would see more benefit under certain circumstances (the CPU test in 3DMark '06). Likewise, the more data that can be RAM cached off the HDD, the faster you can access it, that's much more obvious.
Do also remember that 4 sticks of RAM will mean you have to run at 2T (assuming your motherboard supports that level of timing control) which will reduce the performance of fast RAM.