Unless you're addressing them in parallel, those older 320GB and 500GB drives are definitely going to be slower than a 1TB drive due to areal density - the more tightly compacted the data is over a given area, the faster you can read it. For example, your 500GB drive might have 2x 250GB platters in it while the 1TB drive might have 2x 500GB platters with no change in platter size. What has changed, however, is the density of data on the platters. Not to mention, if you're buying the black edition WD 1TB drive with 6Gb/sec SATA, it's probably the dual-controller model.
With a mechanical hard drive, you're not even going to reach 200MB/sec worth of systained transfer - you might hit 150MB/sec on the outer edge of the platter but that's about it and that's only during sustained sequential reads, something that almost never happens in real-life usage, so 6Gb/sec SATA is really a marketing number. The only home-use drive I have seen to utilize more than a SATA-II (3Gb/sec) connection is the Micron C300 SSD which can hit 350MB/sec sustained sequential reads.
Connecting a SATA-II (3Gb/sec) drive to a SATA-III (6Gb/sec) port can sometimes yield a performance decrease ala the Marvell controllers that are pretty much standard on all new X58/P55 motherboards from Intel. The 6Gb/sec controllers on the AMD 850 Southbridge are hopefully better performing but I haven't really read up on them much.
So just be aware that plugging a SATA-I or SATA-II drive into a SATA-III slot is unlikely to get you any performance increase, if it does, it will be negligible, and the only SATA-III drive on the market that can currently exceed the SATA-II standard is the Micron C300.
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