1) The DH55TC is intended to be used for clarkdale cpu's which have an integrated graphics capability.
If you are into overclockeng, the only OC option is to raise the BCLK value. That is fine for modest overclocks. It is a micro-ATX motherboard.
2) The i7-870 is clocked higher, and has hyperthreading, so it is a better processor than the i5-750.
3) The Asus maximum formula is a full ATX motherboard with a P55 chipset. It has lots of overclocking features if you might need them. But, it is expensive for what you get.
Each combo is about the same price.
If you have multi core enabled apps, then the 870 with the hyperthreading has more compute power.
If you will be gaming, then the graphics card is all important, and the 750 is great for anything.
I would pair it with a more basic P55 motherboard, or even the Intel DH55TC. Unless you have a 2560 x 1600 monitor, a great single graphics card is all you need. No need to pay extra for multi gpu capabilities or enthusiast overclocking capabilities.
If you are on a budget, the Intel H55 board is fine, but I would use a 32 nm clarkdale i3-540 duo : http://www.newegg.com/Product/Product.aspx?Item=N82E168...
and initially use the integrated graphics. That does not preclude you from installing a good graphics card later. You will do better by using the $ you saved on the processor to apply to a better graphics card.
Your gaming will be better with a duo @3.0 and a great graphics card, compared to a fast quad cpu and a mediocre graphics card.