Stephen, you need to dig deeper first.
- Would the entire 'hot' (frequently used) subset of the database fit in RAM if you just upgraded RAM to something larger, like 32 or 64GB?
- Have you checked that your database has the right indexes in place, have you done a basic MySQL performance audit?
About consumer gear:
Using consumer-grade gear like your WD disks in servers is a strongly debated topic. Personally, I think it's a wrong choice in general. But certainly, do not use consumer-grade SSDs like the Intel X-25M (M stands for mainstream). Enterprise-grade SSDs have radically different durability and write endurance goals from consumer SSDs (better wear leveling, more space overprovisioning).
A typical disk setup for a server like yours might be 4 enterprise SAS disks, in RAID10, using a proper RAID controller, with a controller RAM cache buffering all writes, and a battery backup unit for the cache. Such gear isn't exactly cheap, but it is a proven choice.
SSDs do have advantages, and can be substantially faster than a couple of conventional disks in RAID 10 (especially on heavy random reads, assuming the hot dataset doesn't fit in RAM). The Percona team blogs about SSDs and real-life performance with MySQL here.
Anyone have any more up-to-date feedback on whether SSDs are really ready for medium sized businesses?
They are, but IMHO only the enterprise-grade SSDs, and preferably a series of SSDs that have been in production for some time to fix bugs. Good choices right now are the Intel X25-E (Extreme) series, and in a 4-6 months (when they're more mature) the Sandforce 25xx series drives with the enterprise feature set.
If not, how can I scale our db server a little better?
Perhaps you have already done this, but if not: my first suggestion would be to find a MySQL database administrator, and have him do a performance audit of your system. You could very well discover that adding more disk-I/O isn't a cost effective solution for your case.