you biggest problem is the psu 300W. best you can hope for is that a hd7750 will run ok on it. most likely it will since you replace the current card and it's not using a ton more power.
if the normal card fits in the case get that one, if not go for the low profile version.
Thank you for the advice Laviniu, but what exactly is the psu 300w? And I just looked at the ASUS HD7750 , will it be as simple as removing the old card and installing the new one? I appreciate your insight, thanks again.