Need advice on GTX 285 1GB vs gt 640 2GB DDR 3

I have recently aquired a GT 640 2GB DDR 3, but have been using a GTX 285 for ages now, obviously with no DX 11 benefits but very decent speed in most games with decent settings on dx9.

i do not exceed 1680x1050 resolutions on my current display.

Should i change the card for the 640 for dx11 benefits? I suspect that although this is a 2012 card my pc's overall performance will take a hit. If not, what card do you reckon would be the best upgrade at this resolution setting? Thanks for the Help