I bought two Crucial M4 SSD's to run in RAID 0 with my new Macbook Pro. The SSD's arrived before the MBP did, so for grins I stuck it in a USB 3.0 enclosure and hooked it up to my PC. I got about 192MB/s in that enclosure. That's plenty fast, IMO. It's not, however, the maximum speed in the SSD.
In contrast, in the MBP under RAID 0 (which nearly doubles the speed, but reduce reliability) I am seeing speeds around 920MB/s. That's actually faster than SATA III can handle (which is 6gbps, or a little more than 750MB/s. Remember big B, byte, is different than little b, bit). RAID 0 usually nearly doubles the speed of a drive so it's safe to say these drives would operate at around 400+ MB/s on their own in a SATA III setup. So, inside a USB 3.0 enclosure, I'm running at virtually half the speed.
BUT, I'm STILL running faster inside an enclosure, than a platter based drive.
An SSD is quite a performance jump, but something else you could consider is an external RAID 0 enclosure. With a pair of 500GB or 1TB drives, you'll get very fast performance and it will combine the drives capacities (two 500GB drives = 1TB). This will be cheaper than an SSD of a fraction of the size. The caveat though, is that there is greatly reduced reliability. If one of those drives fail, you lose everything. With my MBP, I backup with time machine every day and only store important things on external drives (or in the cloud) until I can get them home and into my desktops RAID 1 system. If you have iTunes match and your entire library is in the cloud, or you have a safe backup of your library on another system, I'd say go for it. If it'll be storing the only copy of files that are important to you, RAID 0 is not the way to go.
If you shop around, you can get a RAID 0 enclosure for about $80 (USB 3.0), and a pair of drives for another $150 for the set. All in all that's a lot more storage for the price of an SSD in the 256GB range (without the enclosure). If your library isn't very big, then the cheaper option is a small (64GBish) SSD in an enclosure. Just decide how much storage you need.
Also, you may notice an increase in speed just going with an external drive anyway. I don't recall if you are using a macbook, but if you are, most if not all are equipped with 5400 RPM drives. These run cool and use less power, but they are slow. A 7200RPM desktop hard drive of a higher capacity (and thus more dense, usually meaning faster) will be much faster than your internal 5400rpm laptop drive.
