I had a bit of a dilemma a while back, in that I needed 500GB storage on my 1.5 G4 Mini (for my lossless library) and needed to retain the internal CD reader (to read in new discs). Also I didn't want to go the external drive route.
As the G4 runs IDE internally and the biggest 2.5" IDE drives available are 320GB, I thought I was a bit stuck on this. Then I found some adapters that allegedly worked and decided to give them a try. These were the Lindy/Startech 2.5" IDE to mSata adapters. They replace an IDE HDD and give you a mSATA slot in it's place. I bought a 128GB old Samsung 840 to test it with as I didn't want to blow a lot on this if it wasn't going to work.
Well it did, and it's belting. It's as quick as the bus can go too, so boot times are really fast. Capacities are up to 1TB also, which is unheard of on IDE and the price of the adapter and mSATA drives (if you're a tight bugger like me) are way, way less than the OWC Legacy SSDs, which are the only other option for this sort of thing.
In the end I ended up getting an Intel mini as my lossless server as it was simply cheaper than buying the 500GB mSATA drive. But still, the trick works and for £60 all up, I managed to get a 128GB SSD into a G4 mini. Which is pretty cool none the less.