Depends on what you do. If you only play classic games and do nothing else get a scandoubler. But if you do Graphics,Word processing or like to surf the net. get yourself a graphics card. A graphics card will speed up your machine more than if you went from an 040 to 060.
Years ago when I brought the pixel 64 for my Amiga A1200 all I had was an 030. People would be blown away at how blindingly fast my A1200 was.
I later brought a B-Vision then a GREX and now I have a Mediator SX with a Voodoo 5500. I would never even dream of going back to AGA and yes I do have a scan doubler. I have used that maybe for an 2 hours total in the last two years.
If you have the cash get a mediator you won't be sorry.
If I had a choice of a PPC Amiga with a scan doubler or an 030 with a graphics card I would go with the 030 every time becuase life is so much sweeter with a graphics card.
Three things I tell everybody with an Amiga to get. SFS,Turboprint,a graphics card. Because using your Amiga without them is just plain silly.
rant over time to get back to work
