Nvidia cards do not mix with AMD cards, at least not as far as crossfire or SLI is concerned. If you can find another 5970, then in most cases, you'd have a lot more power than a single 680, because the 5970 is made of of two 5870's, for a grand total of about 4 5870's.
The problem is that it is probably near impossible to find another 5970 to replace your bad one, so you're likely going to need a new setup all together, or just use 1 of the 5970's.
What is your monitor setup? If it's a single monitor, then your single 5970 should be more than enough, but if you are using eyefinity, then you might want to start over with two 7970's or two 680's.