Ive not used a Mediator, but have a grex1200. While performance of the mediator wasnt as good Im lead to believe that its a more reliable product and it'd be the route Id take if I was ever going to upgrade an a1200 to that extent again. If you want to use your amiga "seriously" RTG is an absolute essential and you'll find it hard to use AGA again after youre used to it. Quite possibley the best upgrade I ever did in terms of adding something to the Amiga OS experience (outshines a simple cpu upgrade, and I say that having bought a bppc card). If a person isnt against the idea of recasing thier a1200 no self respecting amiga user should be without RTG
Being that you also have a ppc card you'll open up new doors to what you can do with your amiga, albiet with the limited ppc software available, although even 680x0 software will get a kick in the pants.
Now as for graphics card, a Voodoo3 is probably your best option still. The Radeon cards that will work, while technically superior to the Voodoo3's dont have 3d acceleration, and the drivers arent nearly as mature, so the end results will favor the Voodoo3 on average. Sound card wise I dont think matters too much, as the hardware is too weak still to do much more than the absolute basics, and anything will be somewhat of an upgrade anyway. I personally just used an old sb 128.
I absolutely recommend something like a Mediator to someone like your good self who uses thier Amiga the vast majority of the time. It really can breathe new life into a classic machine. It also gives you the opportunity to use OS4.x if that's of any interest to you (although personally I think it's crazy to do so,.... too underpowered for "modern" use as OS4.x is intended, and you lose compatibility with anything requiring custom chipset, even if it's there).
