I've read that connecting the Amiga's composite output to the PCI TV card will work. Never tried it first hand, though, and video quality through that port is pretty bad.
I use a basic VGA switchbox to flip between CVision and AGA modes. It's selling point is that it's far cheaper (I paid $15US) and more compatible than a KVM (no PS/2 nonsense). Bad news is that there is some signal degredation. It's only really noticable above 1024x768, though, and I never use resolutions that high on my Amigas. The effects can also be minimized by fiddling with CGXMode/P96Mode.
