I don't know if anyone posted about this before, but at Storage Review they apparently run a simple I/O access test with WoW whenever they test a new hard drive, and they've got a relatively small database of results. As expected, two SSDs are far and away the fastest, but one comparison caught my eye. The following lists the model and some specs, along with the I/O-per-second scores (for reference, the two MTRON SSDs scored 3,333 and 3,030):

Samsung Spinpoint F1 with NCQ (1000 GB SATA) - 787

Samsung SpinPoint T166 with NCQ (500 GB SATA) - 515

Aside from price (~$100 for the 1TB vs ~$60 for the 500GB) and size, the primary difference between the two drives is the size of the cache buffer. 32MB for the 1TB drive, 16MB for the 500GB drive. Doubling the cache size meant the difference between a drive that posted one of the highest scores (the second-highest for a 7,200 RPM drive) and a drive that languished much farther down on the list. The only 7,200 RPM SATA drive that beat it was the Hitachi Deskstar E7K1000... which has a 32MB buffer.

It makes sense, if access times are so important to WoW. The memory buffers on a hard drive are much, much faster than accessing data directly from the platters (which is the whole reason for the cache in the first place) and thus bigger works out better for a game moving so much data at a moment's notice. Consider that you are looking at a cost of $100 per drive for performance rivaling 10k/15k Ultra320 SCSI drives. On the other hand, if you have money to burn then you are looking at possibly a 3.5x to 4.5x performance increase from using an SSD (or even bigger, depending on how well other brands perform).

EDIT: Hmm, looking further into the list, Seagate's ES.2 1TB drive with a 32MB buffer places pretty low on the chart. It uses 250GB platters versus 334GB platters on the Samsung Spinpoint. Very curious...