My new Alienware pseudo-Dell computer arrived, I'm still busy installing everything and testing. But this site has a very good explanation why I shouldn't have bought 4 GB of RAM for it. 32 bit Vista only has 4 GB of address space, and a large chunk of it is taken up by the address space needed for the PCI bus, other components, and most importantly your video card memory. So due to my 640 MB Geforce 8800 GTS, Vista can only access 2814 MB on my machine. Doh!
I should complain to Alienware, why are they selling Vista machines with 4 GB, if its impossible to use that much RAM?