if you have pci-e 2.0 motherboard then get a gtx 660ti if you can, if not get h77 based mother board first. like asrock H77 Pro4/MVP this is a pci-e 3.0 motherbaord. This wont overclock "That Much" and since you prefer to turn it off then it's not a problem I guess,.
*get a 1080p monitor for better gaming experience
*look also in the amd side, like the Radeon HD 7870,.
the standard 660 ( non ti ) would be a big upgrade over what you have now. more than enough to run everything at max settings at your current resolution although, if upgrading monitor to 1080, you might have to lower some settings in some games depending on the particular demands.