I'll second the SSD. It won't improve FPS in games, but it will improve load times dramatically. You ought to be able to find a decent non-Sandforce 120GB-128GB drive in your price range (e.g. Samsung 830, 840, Crucial M4). Adding RAM likely won't help. You might add a decent air cooler like a Xigmatek Gaia ($20) and overclock your CPU. Hitting 3.8GHz is not unreasonable; you might get more. I don't think it is worth Crossfiring two low-mid range cards like the HD7770 due to microstutter and potential driver issues overcoming any benefits. You could sell or return it for a stronger card (e.g. HD7850), but if it were me I'd get the SSD.
Thirded SSD. What does that tell you? Get an SSD ;P