what about nxt coin? you don't need any GPU's to collect fees, just a lot of nxt but I don't think 300$ will get you a whole lot of nxt maybe just around 5000 (not sure exactly but you would probably need a lot more nxt in order to make it worth it for you.
I would personally recommend Peercoin for a safe long-term investment in the proof-of-stake concept. There is a significant risk of Nextcoin being a temporary fad, while Peercoin has always been the third largest by market cap.