In a nutshell, Vista Home Premium is good for 16GB, Vista Basic is good for 8GB, while the Enterprise, Business and Ultimate versions are good for 128GB.
The odds of seeing any benefit from 6 or 8GB of memory are pretty slim outside of A/V processing, server and rendering sort of applications. Really, there are only a couple games that need more 2GB, let alone any higher.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133
I upgraded from 4 to 8 and saw a decent speed increase using Vista Business.
I ordered a system w/ 16 GB of RAM (Phenom 9950 BE, 790GX, 16 GB, HD 4830, 640 GB HDD, Vista HP) for a friend, so we'll see how that goes once the parts arrived (NCIX is taking FOREVER to process the damn order). NCIX had 4 GB sticks for $49 each, so my friend said to just max it out.