280x is better for 2 monitor, nice frame rate, nice settings. still you need a 650w+ to do that. In the end, you would have to upgrade your psu before any gpu upgrade.
both of them(250 and 7750) performs almost the same but the later might get a slight better frame rate because it got a more stable driver whereas the 250 is still new(but could gone better than 7750 when a driver optimized for it comes out). IMO choose whichever that is cheaper
doubt anyone would heard it spinning at <30% unless you stick your ear to it. practically not useful at all and you had better no charge for extra just so because it can run at 65c at zero fan speed
maybe your card have some power connection problem within its pcb or voltage prob in it. take it to service. dont game on it if not it might get too hot where it might damage your other component