One of the problems with Vista is that its prefetch (superfetch) algorithm was way too aggressive. So give it 6GB of ram, and it's going to sit there thrashing the the crap out of the disk preloading all your programs/data into all that memory. Even while you're trying to use the machine. One of the things they did in Windows 7 was dial it back.
I ran Vista on a P4 with 1.25GB of ram for several years, and it was surprisingly usable. My theory is that it was enough ram for the OS and a few programs but didn't really leave enough for superfetch to play around with. I also tried installing Vista on a P3 laptop with 512MB of ram. That... didn't work as well.