Full disclosure: I'm a hardware nerd.
I was recently given a single radeon hd 7970 card. My current system has 2 linked radeon 5700s. My question: would the 7970 outperform the two 5700s? Bonus question - could i link the 7970 with one of the 5700, and would that be the better option? Appreciate it.
7970 offers more performance for power.
has less idle and long idle power consumption due to improved power management e.g. powertune and zero core power. it will use less power in idle mode compared to 2x 5770.
since you got it for free (i am assuming), performance per dollar is unbeatable.
no microstuttering (if you're susceptible to that) and hassle with crossfire profiles and driver issues.
less card = less heat and more airflow.
7900 won't cfx with 5700. they belong to different underlying architecture and different number series. besides, amd will never allow a high end card cfx with a lower end card.
you get 3 gb vram for your gpu - good for cranking up AA and other gfx settings. you only got 1 gb (or 2gb if you bought the 2gb versions of 5770) for the cfx 5770s.
much improved gpu computing capability with 7970's gcn arch.
2x 5770 performs up to gtx 570-580 level heavily depending on games. the 7970 more or less outperforms the gtx 580.
now it depends on your pc's specs if it can support the 7970 and take full advantage of it (including your psu).
hd 7970 will be the best choice because its of newer tech with good performance and some games doesnt support cf and sometime there will be issues with crossfire so it will be better to go with single high end card and even the hd 7970 performs better in higher resolution.