The Samsung F3 is good as it uses 500GB platters. The WD one still uses 250GB ones, making it extremely slow. Either pick the Samsung F3 or the Seagate 7200.12 as they're the only ones using 500GB platters.
What brand is your PSU? A good PSU (read: a big brand name one like Antec, Corsair, PC Power&Cooling, OCZ,..) can be reused, a cheap one (Rosewill and other ones) will cause trouble. As the beating heart of the PC, it's important to have a good PSU.
Good point on the PSU. I was just assuming it would be okay.
I am currently using a 3XS Systems 400U-SPTP 550w PSU. I have no idea of how good a make this is as I cant find any reviews. As 3XS are the company associated with Scan.co.uk, I wouldnt be surprised if it is a rebranded generic.
So far it seems to stay cool no matter what load I put it under, it's under 2 years old, and I havnt had any warnings on power consumption like my last PSU. So I am *hoping* that this one will be ok.