PCI-E expands their bandwidth capabilities before the bus gets saturated. Think of it as upgrading a power grid for a city before the city expands beyond the capability of the grid.
Current cards do not saturate the 3.0 16x bus, but when you run SLI or Crossfire 680/Titan or 7970s on a Z77 board you want PCI-E 3.0 because it drops to dual 8x. Dual PCI-E 3.0 8x is the same bandwidth as 2.0 16x. There is no benifit of putting lower end cards in a 3.0 slot because low end cards do not saturate a 2.0 16x slot. They are only 3.0 because the chip tech is based on 3.0.
PCI-E was set as a standard for backwards compatibility. You can put the top of top end cards in a low end 16x 1.0a slot if you wanted to. It would hold back the performance, but would still run regardless. In your case, putting that video card in that computer will be 100% fine performance wise. THe 7750 is a low end card.