So, this is to give an idea on HD 7770 crossfire performance:
http://www.techpowerup.com/reviews/HIS/HD_7750_7770_CrossFire/22.html
Considering Radeon 7000 series have been around for almost two years, I would personally choose either Radeon R7/9 series or Nvidia GTX 7xx. Now, in terms of performance per dollar, it climbs up (as in you get more performance per dollar) up until 350 dollars, give or take. Which means the best value card (or one of the best) is undoubtedly GTX 770, however it is priced ~ 140 dollars above your limit:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125463 (340)
To provide you with a general comparison among the cards currently available, check this out:
http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32884-7.html
It's a graphics comparison chart, and Tom's recommends you upgrade your GPU only if it's atleast three tiers above the one you've got right now. Which would mean you should go with no less than GTX 650Ti or HD 7850. Now the issue I see here is that both these models are dated, and the newest additions to Nvidia and AMD's line ups, like 750/750Ti or R9 270/R7 xxx series are not included, which would be close to your budget and provide a very reasonable performance. Some of them reviewed here on Tom's:
http://www.tomshardware.com/reviews/geforce-gtx-750-ti-review,3750-13.html GTX 750Ti, not yet released, reference price 150 dollars
R9 270 is over 200 dollars, however (270 on Newegg) (just mentioning it for reference purposes)
Other in line on AMD's list is R7 260x which is a weaker performer. however its within price range:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125486
http://www.tomshardware.co.uk/radeon-r9-280x-r9-270x-r7-260x,review-32796-11.html review on Tom's
Lastly, if you are willing to wait a bit to see if the price drops OR you might be able to save a little bit extra, my main suggestion would be going for GTX 760.
It's performance is included in this Radeon cards review for comparison:
http://www.tomshardware.co.uk/radeon-r9-280x-r9-270x-r7-260x,review-32796-11.html
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130932 250 dollars.
It may seem that I give preference to Nvidia over Radeon, and you would be right. Thing with radeon new R series is, these cards are renamed 7000 series, if with more memory and increased clock speeds, yet they still are based on dated technology. Whereas Nvidia's cards are running on the recent Kipler technology, and the forthcoming 750 Ti will employ Maxwell, which is the newest in their arsenal. In addition, when it comes to price/performance, I have found that Nvidia cards do provide best ratios, especially when you consider their trustworthiness. Not to mention that some of the Radeon R series cards have their prices inflated by outside factors.
Bottom line, these fancy words like Kipler and Maxwell don't mean much to me as I'm by no means an expert in the area - it just gives you a general idea how old the technology running inside the card is, and what money they are asking for it.
So there you have it, I hope this made some sense and will be atleast of some assistance to you
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