the required power supply for 7750 is 450w, mine is 400w. will it work?
and what about the pci as mine says pci x16.
Yes, yes. HD 7750s actually run just fine on 220w PSUs. The card never draws over 40w during gaming unless you overclock it. I doubt your whole system including this card will pull more than 100w during games. And all versions of PCIe x16 are forward and backward compatible.
Like I said, pop it in and play.