2.2) Buy top-of the line AMD GPU - would probably double my hashrate, but would leave my current NVidia useless - and somehow I've grown fond of the Nvdia and cudaminer :p
Why would it leave the NVidia useless?
There´s no problem combining amd and nvidia cards in one rig.
you just need to install both graphics drivers and run two different miners.
Or am i misinterpreting something here?