with a 300w PSU I would only recommend the HD 6670
as far as PSU upgrade what is your budget?
also is the tower a full size or a slimline?
full size is about 7.5 inches wide
if it a full size then usually a ATX supply should fit
Antec.Seasonic.XFX.Corsair are the "Big Four" of powersupplies
if you can measure your PSU from the back in relation to power plug inlet or post pics that would help