There isn't a 500gb F1, only 750gb and 1tb. There is a 640gb SE16 from WD that is wicked fast, you'll have to see if the price is right over there in Britain.
You'll need more than 12gb I think, probably more like 30gb or so, I'm not sure exactly how much vista takes. You don't even really need to partition it, it won't do anything for speed, it just may make it easier to organize. And I think 4gb does show a pretty decent advantage over 2gb. Things load faster, and just generally run smoother. Vista especially loves the extra ram.
Edit: Also, I also would get a bigger hard drive, but thats b/c I know I want to put more stuff on there. If your not going to use the space, don't worry about that. If your just going for speed, maybe consider saving up for a veloci-raptor, or setting up a couple 320gb 7200.11s in RAID. Otherwise if your looking for space and speed, I think the WD 640gb SE16 or the F1 will be your best bet, but it depends on the price.