With 18MB of ram you should try to use 256colours screens and maybe a 16bit one. But try to keep the number of >8bit screens down.
You A3000 will feel MUCH faster with a CV64, regardless of you using a 030 or 060.
CV64 includes the ROXXLER chip that does planar to chunky conversions so you don't have to waste your cpu doing that.
CV64 is one of the fastest Zorro3 cards. Its bus is faster than Picasso4, CV3D and Prometheus. Its monitor-switcher makes it very comfortable to use.
If you have access toa cheap CV64 get it, it's a very nice card with monitor switcher, unlike voodoo3 etc, that will force you to buy crappy external monitor switchers. A CV64 monitorswitcher is automatic and truly comfortable.
I would get a CV64 for a 030 A3000 but I wouldn't waste my money buying a Prometheus or Mediator unless you get it 2nd hand very cheap or plan to expand your A3000 a lot. A CV64 is far more comfortable thanks to its monitor switcher and it's very fast. In fact, its bus speed is faster than Prometheus/Mediator.
The 1st amiga gfx card I had was a GVP Spectrum, the 2nd one a CV64, the 3rd one a CV3D (bad change), the 4th one a Picasso4 (I still have it, I love it), the 5th one a Prometheus+Voodoo3 and the 6th one a Mediator4000Di.
CV64 had a clear image, was fast and comfortable. It will be a joy to use regardless of your machine being upgraded or not.
@amigakit:
sorry, but OpenPCI doesn't work very well with Prometheus because DMA stuff is not finished. E3B will probably publish an update next year. I own both Mediator4000Di and Prometheus and Prometheus works very well with Voodoo3+RTL8029 but that's all. Mediator works better if you plan to use TV, USB or Sound. That may change when E3B finishes his Prometheus firmware upgrade, but until that day don't speak about OpenPCI on Prometheus too loud ;-)