No, a cheap GPU won't really help either way. If you're just doing non-intensive stuff on your other monitors then your 970 won't be taxed, if you are doing intensive stuff then a cheap GPU won't help.

I personally wouldn't get a monitor that big, but if you think that's ok for you then go for it. IMO both those monitors are expensive for what they are, but I'm kind of picky about monitors. If I were you (which I'm not) I'd look for some local dell ultrasharp panels to use as secondaries (IMO they are the best non-primary displays). Right now I'm using a single FP1355, so yeah, this kind of topic is mainly personal opinion - unless you want to optimize a specific characteristic or set of characteristics.