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?
There is no problem using both amd and nvidia cards in a single computer. it works.
also even if the perf. of the amd are better that doesn't the other card is useless...
and yes you'll need a new psu.
But quite frankly I really prefer cudaminer to cgiminer.
Now there is two way to see things, if the computer is only mining you might put whatever you want in it, but if it is also used for gaming... I think it would make more sense to stay in nvidia and do some sli (I regret a bit to not have gone for the second gtx780ti for that... however I didn't play since I started mining... I have at least to try the R9 on bf4, it is supposed to outperformed the 780ti...)