you will see a performance drop of less than 1%. the 7750 isnt all that powerful and even if it is gtx690, you wont see much of a drop anyways. video cards are not even using up all the bandwidth provided by pci-e 2.
therefore your video card can run nicely in a pci-e 1 or 2 system. 400w should be more than enough. the 7750 doesnt need any auxilary power plugs