7750 is a low-end card, but you can lower graphical settings to make up for a weak GPU. If your CPU isn't up to the task, there's nothing you can do.
Buying a new GPU would probably not improve your framerates much (your CPU will prevent that), but you could run higher graphical settings with the same framerates.
Buying a new CPU would allow you to get much higher framerates, but you'd need to lower your graphical settings most likely.
$80 isn't going to get you very far, unfortunately. PC gaming is a relatively expensive hobby. $80 will only really buy you another HD7750 equivalent card, and $80 will not buy ANY motherboard + processor + RAM combo that's worth owning.