The card you currently have is about $130, so I suppose you could just get another 7770. The only problem with upgrading cards at this level. you will experience a lot of bottlenecks because you will only have 1gb ram.
No, the only cards that work in cfx with your card are the 7750 and the 7770. There is going to be ram bottlenecks in a lot of games because two 7770's will have more power than 1gb of ram can provide. With your budget, that is literally your only option. My old 560ti got ram bottlenecks, and I am guessing two 7770's have more processing power than 1 560ti. So your two options are to wait until you can afford something like a 7870, which is only $60 above your budget, or you can get another 7770. The second option will definitely boost performance, but I can guarantee there will be bottlenecks.
i own the 7870 hawk you wont be dissapointed 1100mhz stock with alot of headroom if u wanna overclock it later and its eye candy to look at aswell all components on it are strong mine stock never gets above 60c very good card from msi