You'll love your new SSD hardrive but Diablo is sort of a basic game, you won't notice much difference. I have a Plextor SSD Sata 3 that I am using on a Sata2 motherboard but it's still way better than a regular hardrive by a long shot.
You're right that it's basic. It's so basic that it can't even load data to memory while loading a sequence. Instead while playing it will load everything directly off the hard drive, causing horrible lag spikes no matter how high end your system is. Until you get an SSD, of course.
Load times, such as when you first enter a map or start up the game will virtually disappear with an SSD compared to a 7200rpm.
Load times in Diablo 3 aren't the problem. The problem is the lag spikes while playing, and yes, it's worse at the beginning of a session, but it never entirely disappears until you get an SSD or make use of a ramdisk.