My sata WD 320gb capacity says 319.73 GB (319,728,959,488 bytes).
Basically, unless you're running snow leopard, which you're not because it's not available for that machine, 320GB means 320 billion bytes, i.e. 320 000 000 000 bytes. Gigabyte is being used in the sense Giga is used in SI units, i.e. as a power of 10.
Computers generally measure things in powers of 2, so your Mac software measures a gigabyte as 1024 x 1024 x 1024 bytes. This should more accurately be called a Gibibyte, but it is in common use on many computing platforms. This is why Apple have changed their reporting on the newer operating system, to help avoid confusion.
The bottom line is (until snow leopard), operating systems (Windows, Linux, Mac, any one you like) report capacity in binary gigabytes (i.e. gibibytes) which are larger than SI gigabytes. Hard disk manufacturers report sizes in SI gigabytes, and end up therefore with a larger figure.
None of your space is missing - your drive is 320GB where GB = 1000000000 bytes. However any operating system available for that computer will measure in GiB (1073741824 bytes), thus you will see a figure somewhere around 290-297.
You will also 'lose' a small amount of space due to formatting and other information the computer uses to actually know where files are on the disk.
This will also have been the case on your previous hard drive, and almost any other previous computer, it's just that the difference between the 2 measurements gets larger and more noticeable as capacity goes up. Hence, for example, my 1TB hard disk shows as 931GB on a computer. Your 60GB drive would also have been smaller, but by a much less noticeable amount.
You still have tonnes of space - be happy!