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As far as I know, the 640GB WD drives are still dual-platter. Also, the "Black" version of the drives should be geared toward slightly faster performance than the normal "Blue" version, but at the expense of slightly more noise and heat, in addition to a typically higher price. They also come with a 5 year warranty instead of 3, though I doubt their reliability is much different, and you should make sure you back up important data either way. I've been using a 640GB Blue drive (though it wasn't called "Blue" at the time) for a few years, and it's performed rather well. I can't really provide any personal suggestion on which among the current 7200 RPM single platter drives is quietest though, aside from what could already be found in reviews.

Another option, if you don't need much space and are willing to pay more, would be to look into an SSD, which should be completely silent, and if it's a decent one, much faster than any hard drive.

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You may also want to consider a 2.5" drive like the WD Scorpio Black. If you have an easy way to suspend it, it should perform as well as any other "quiet-oriented" 3.5in drive running at 7200rpm.

Might cost you a little extra though...

Note that the F4, even that 320GB version, is an Advanced Format Drive, meaning it is a hassle to use in Windows XP. I tried to find this out before buying one but no source I looked at online had the information. The drive I received had a warning printed on the label about it and a link to a program they have to "align" the partitions. Pictures of this drive online don't have that message on the label.

Anyway, it only affects people like me using XP but I thought I'd bring it up. From what I've since read it's also a problem with a number of NAS devices.

The F3 isn't AFT and that's what I ended up with after I tossed the F4.

I've been using a Samsung F2 drive for years and it is as close as you'll ever get to silent. The F3 was reviewed as being equally quiet.