Actually, it is 298GB. They do the usual marketing ploy of calling it 320GB, but they are actually using the definition of 1GB=1 billion bytes. They say this in the fine print on the box. So, they call 320GB 320,000,000,000 bytes, which is the actual size. But it's not 320GB, because 1 kilobyte is 1024 bytes, 1 megabyte is 1024 KB, 1 gigabyte is 1024 MB, and 1 terabyte is 1024 GB. So 320,000,000,000 / 1024^3 (cubed) is 298.02 GB. However, ironically they use the true definition for a MB for the cache (1,048,576 bytes), which is 8MB, so that is the real value. But if they use the 1 billion byte definition, they get to use a larger number for the capacity. Kind of like how things always seem to cost $99.99, or $49.99, or $9.99, instead of $100, $50, or $10, because it looks like a lower amount, though there's no significant difference (nobody cares about one cent). That's capitalism for you, heh.
Well, since this has only in the past few years made an appreciable difference, the SI unit people have clarified the whole GB=1,000,000,000 bytes thing:
http://en.wikipedia.org/wiki/GibibyteAnd so officially, a Gigabyte is 1,000,000,000 (10^9) bytes, just like a Gigahertz is 1,000,000,000 hertz. A Gibibyte is the binary version (2^30), or 1,073,741,824 bytes. So that hard drive is actually 320 Gigabytes, or 298 Gibibytes.
As an aside, I don't like this system, I have the binary system too well ingrained in my mind from >20 years working with it... But it's something I need to be aware of, as with increasing sizes of storage it's only gonna become a bigger issue.
Back on topic: I also tried a WD Scorpio 60GB in 2 A1200s - no issues with the standard PSU, CD-ROM and Blizzard 1230...