If you get the Z170 board then you have to get a new CPU and DDR4 ram. No matter what if you change the board the CPU has to be replaced at the very least.
the chipset options for that i7-2600 may be limited to whats for sale. asus and gigabyte are both top quality mfg's, asrock is ok and has several mbd's for sale. since you don't have the unlocked i7-2600k, the z77 chipset may only be good if you plan on dual video cards. the h77 doesn't support overclocking or dual video cards. the b75 mbd's may have one sata 6gb ports and no raid.
unless you need the features of a quadro for an application outside of adobe's software, most people here just use a nvidia gtx with premiere. the quadro m4000 is an underclocked gtx 970. there are several gtx models that are available in short cards, made for htpc type cases, including some gtx 970's. if that's all you need to upgrade your old dell system, then you don't have to rebuild the whole computer with all these parts. if the dell psu won't upgrade or power the gtx 970, there are some gtx 750's that don't need extra pcie power cables. the gtx 750 is half the performance, but uses alot less watts and may still be fine for 1080p projects. if you do stick with the dell psu, you will want to check its rated power watts vs the video card requirements, to make sure you aren't overloading the psu.