This is one of those questions where the only valid (enough) answer is "it depends ...".
We can begin with your question about whether using a Crucial M4 SSD is "worth it" in your MacBook.
No, it is not.
As you pointed out, your MB only supports SATA I with a max throughput of 1.5 Gbit/s. It makes no kind of cost/benefit sense to pay the premium for a recent SATA III capable SSD when your MB will only be able to use it at a fraction of the speeds it is capable of.
Another limiting factor is that your MB uses a 667 MHz Front Side Bus (FSB). Strictly speaking this is a memory (RAM) to CPU throughput limitation, not SATA. But I bring it up because it is a factor limiting your MB's overall maximum performance. That is, it is another reason you would not enjoy the full benefit of a SATA III capable SSD. Your MB is unlikely to be able to move data between RAM and the CPU fast enough to stress even your SATA I controller's throughput.
So if you ultimately do decide to move to an SSD, I would suggest you consider older SATA II models which would be a better fit to the throughput of your MB. The older SSDs should also be relatively less expensive.
Another concern with using an SSD in a MB is if your version of OS X supports the
TRIM command. My understanding is that you would need either Snow Leopard (10.6.8) or Lion (10.7). Also, since Apple's OS X only supports
TRIM for Apple supplied SSDs, you may still have to jump through hoops to enable
TRIM for the SSD you choose to use.
Of course, not having support for the
TRIM command does not make your SSD worthless. However, it can mean that, over time, you will suffer a significant degradation in your SSD's write speed. So it is something you might want to look into further before deciding whether or not to move to an SSD.
Finally, an SSD is not really a good choice for storing large amounts of data such as music files. You would probably be better served using a traditional HDD with a larger capacity.
Another possible alternative might be using Seagate's 750GB hybrid HDD, the Momentus XT. While you will never see the SSD performance from a hybrid, you may get enough of a boost to make you happier using your MB. And you would also have the extra storage space for storing large (music) files.