You aren't going to find many single slot GPU's unless you go really cheap (like GT630 or R7 250). On the best bang for buck subject, R7 265 or GTX750Ti right now, sure they're not going to keep up with cards capable of ultra settings in some games, but they play most games with high settings above 60fps and require less power.
I went from a 560Ti to a 750Ti in one of my systems and it's a decent step up, not a large step up but it's noticeable and it consumes significantly less power (560Ti sucks over 200W under load while 750Ti is still under 100W under load) plus I gain the Shadowplay feature which isn't supported on the 560Ti.