Up to you, but why not just buy both from Newegg? My experience with MicroCenter is that they sell some stuff cheap just to get you in the door (mobos and CPUs) and jack up the price on other stuff to make up for it. In this case the power supply, seriously, all of their Corsair TX series PSUs are substantially more expensive from MC than newegg. You might find the video card isn't any cheaper there either.
I'd honestly go at least 660TI 7870 to really justify the upgrade. Yea, the 6870s/560 TIs are better, but hard to really justify throwing the money down. Unless you can find another use for your 550 TI.