Oh no no no no no no no.
You can't just dual boot into Mac OS with a PPC accelerator! Using the same logic you should be able to dual boot into 68k Mac OS and STOS with your current system which I am sure you know is not possible. Emulation is an option however but if you want to use PPC Mac software and OS 9 then you'll need a PPC card (obviously) which costs a lot, can be unreliable and is difficult to source. You'll also need a copy of iFusion which again is difficult to source and I've heard it's very difficult to get to grips with.
My suggestion would be to go for an '060 accelerator as it will easily cope with your needs. With it you can use Shapeshifter (it's free - simply download it from Aminet), which absolutely flies when using an '060, for 68k Mac emulation. As far as I can recall it is capable of running anything up to Mac OS 8.1 and I currently have Mac OS 8.0 installed on its own partition on my A4000 which runs Netscape very nicely.
You also said you'd like to play music and I'm assuming you're referring to MP3 files. For this an '060 (or better, PPC) is a necessity. I use Songplayer on my A4000 and the sound quality is very good indeed but in order to get the best quality you need to be using a 30khz screenmode. Seeing as you have said you want to buy a Mediator board (and I assume a graphics card to accompany it) this won't be a problem for you.
Lastly I'd like to offer you my tuppence worth if I may; I wouldn't bother going for a PPC card based on what you want your Amiga to do as an '060 will be fine and infinitely cheaper. Maybe if you see a cheap '040/25 with 603e/160 minus the SCSI interface it may be a similar price to an '060 (and if it is then buy it!) but I reckon you'll be waiting a long time in order to pick one up that cheap.
