Better Video card Hp Slimline?

Is there a better video card i can put into a hp slimline? A regular card wont work because its to big, and i cant run any games with the one i have|||Look for the words "low profile" while you're searching. It's their term for smaller cards. But I am not sure even those would fit. I almost bought the slimline, til I saw somewhere that it's almost un-upgradeable.