You can always move the components to a bigger case, yes.
The best card for the stock 320W PSU (if that is what you actually have), would be the GTX 750 Ti low-profile. But be aware, that HP motherboards/BIOSs have been notorious for having problems with cards that are not on their QVL (qualified vender list). The new Maxwell-equipped GTX 750 Ti is an example. I'd check with the folks at HP or the HP forum to see what the chances of success are.