Here's how I did it with my trusty Pismo (PowerBook 2000). I'm really pleased with the benefits. It also has a G4 upgrade; between the G4 (hotter than the stock G3) and upgraded hard drive (hotter than the stock pokey 20GB hard drive), the fan was coming on all the time. Now, the fan is usually quiet, there is no hard drive whine, and it runs more smoothly. Doesn't magically make it screaming fast, but it's noticeably better. And best of all, very affordable.
On Amazon, there is a product described as "mSATA SSD to 44 Pin IDE Adapter as 2.5 Inch IDE HDD 5 Volt" (current price about $12)
http://www.amazon.com/gp/product/B009BA8V4M
Then, on eBay, I found a used mSATA SSD. Pismo maxes out at 128GB on its internal IDE bus, so that's what I got precisely. It was described as "Samsung MZ-MPC1280/000 128 GB SSD mSATA 1.8" Solid State Drive." A modern PC needs more storage than 128GB, but it's perfect (and cheap) for my Pismo.
I wrote a customer review for the adapter, on Amazon (August 2015). I'll just include it below, instead of rewriting the details here.
This is an excellent product at a great price. It functions as expected, which is saying a lot when trying to mix old tech with new tech.
I use mine with an old Apple PowerBook (the year 2000 "Pismo" model). It originally came with an IDE/ATA 2.5-inch hard drive. I was already using a 32GB SDXC flash card (with a different adapter) instead of the hard drive. That worked well as a cost-effective replacement for this older type of laptop hard drive, but its performance was not noticeably better (or worse) compared to a mechanical hard drive. The main advantages were (lack of) heat, noise, and weight.
Using an mSATA SSD with this adapter has all of those advantages, AND performance IS noticeably better. Plus overall cost is even lower (per GB); I'm using a previously used 128GB mSATA card (Samsung branded) that I bought on eBay, which appear to be "pulls" from laptops that were upgraded to higher storage. Since my old PowerBook can only recognize up to 128GB internally, it's a "good fit." It cost be just over $40, or about $55 TOTAL cost including this adapter. A high-quality 128GB SDXC card (recently much cheaper) costs significantly more than that (without an adapter). OWC sells a "specialty" SSD with the 2.5-inch drive form-factor that has a built-in IDE/ATA connector (instead of SATA), but it costs more than twice as much for 120GB. Therefore, using an mSATA card (which is designed to be a laptop drive) with this adapter is a bargain.
I took one star off because of a minor design flaw. There is a component on the adapter's board that sticks up too far. When the mSATA card is "folded" down against the adapter's board and secured in place (with two screws), this component interferes, at least in my case (it may not be an issue with other mSATA cards). Fortunately, the difference is about 1mm and there is good contact between mSATA card's connector and the adapter's socket. However, if I did not notice this issue and tried to tighten the two screws completely, I would physically break the mSATA card. My suggestion is to barely screw in the two screws initially. Then, slowly tighten the two screws (alternating) until the mSATA card makes contact with the component on adapter, and stop tightening there. If this issue does not exist for your situation, tighten all the way.
NOTE: mSATA card and adapter only take up a fraction of the space used by a 2.5-inch hard drive. You'll need to devise a way to hold it in place, since the existing mounting method for hard drive is useless. I used a few sheets of foam packing material, cut to appropriate size.
I still use this Mac next to where I sit to watch TV. Works fine for email, and "light" web browsing (thanks to the TenFourFox browser).