The issue is not with your processor or the ram but rather by the motherboard and it's accompanying BIOS. It's already been established that prebuilt system's have a limiter installed by the Manufacturer to safeguard the company and the end user and in this case the lack of a UEFI BIOS environment will prevent you from dropping in a current gen GPU.
If you must look at a GPU upgrade, provided your PSU can power the pseudo upgrade, look at a GTX600 series
(or equivalent).
In retrospect, if you're looking at a GPU upgrade for gaming, then at 1080p gaming or less you're best served with the GTX650 at most.