The HD7770 in games is roughly between 55% and 70% of the GTX660 (it varies by the game). The i5-3570K versus your 2-core CPU will vary quiet a bit by game as well (Crysis 3 is CPU intensive).
You're going to be spending over $500 however to upgrade your system:
3) Power Supply
With such an expensive upgrade, you may wish to ask yourself if it's even worth it (if it's only for Crysis 3 I'd personally say no), and if so maybe you should spend a LITTLE MORE and get a better graphics card?
It may be worth investigating what $300 or so will get you from AMD because they include Crysis 3 with some of their cards as well: http://www.newegg.com/Product/Product.aspx?Item=N82E16814202026
The above card has Crysis 3 and Bioshock Infinite included. Since it costs $290 (after MIR) the actual card costs LESS THAN $200.
HD7950 vs GTX660:
Performance is similar, but the benchmarks seem to indicate the HD7950 on average beats the GTX660. You'll have to look at the Pros and Cons more closely I guess as it's not completely cut and dry.
- over $500 to upgrade
- HD7950 recommended