Is it worth it to upgrade a mid-2007 MacBook (MB061LL/A) to use an SSD? I would sincerely love a new MacBook, but I can't justify spending the prices that are being touted by Apple what with life to pay for and everything.  I upgraded my memory to 2GB a while ago, but am thinking of now upgrading the HD to a SSD seeing as the MacBook is still performing OK, just a bit lagging behind the times.

For the record, my MacBook is the 2.0GHz Intel Core 2 Duo (13-inch White) MB061LL/A version.  I have upgraded the HDD 2 or 3 times since this purchase in 2008.  The current RPM is 5400, but I have just found out I was missold as I have never knowingly bought a HDD other than 7200rpm (!).

Crucial recommends their 256GB Crucial M4 2.5-inch SATA 6GB/s SSD. But I know that the SATA controller in my model is only 1.5Gb/s. So is this actually worth the upgrade?

To give some idea on usage, I mainly surf the internet for long periods so no real heavy usage apart from spinning or streaming a film now and again. But I will now be looking to use this as my main music recording computer due to my dedicated PC dying.

Thanks in advance
Yes,
... because there are other advantages besides the faster access times.

SSDs work inaudible
SSDs are shock-resitant
SSDs use less energy but the total impact is very little...
SSDs max out the SATA 1 throughput, which your old HDD will probably not.

Also, you might get a SSD and then use it in your newer Mac which you will buy in a few years. Make sure to get a SSD with a SATA 3 interface - which is backward compatible to SATA 1.
What about TRIM? The current SSDs have very efficient garbage collection which make the need for TRIM dispensable. But as such performances largely differ depending on the hardware manufacturer, make sure to study performance reviews of the SSD which you'll consider.
I've written some more in this post.