Should I buy a second 5770 for crossfire? Or should I just buy a 7xxx?

I don't think its good to get another 5770 for crossfire.

you should just wait and save up some more money.

I have no budget. I just want to see what would give the best performance per dollar ratio.

then you need to say what resolution and what games you play as well as probably your cpu and psu.

value is kinda a relative thing, people who game at 1600p say that the 590 and 6990 are good value while its not for the average person.

Fair enough. Budget of $200. Resolution and anti-aliasing doesn't matter, but I would like to be able to run most games on high settings in the next 1-2 years.

then I'd say something like a 6950 or 560 ti to play on high for the next couple of years but you'd need to spend a bit more than $200

at about 200 you'd be much better not going crossfire. A 6870 would be a modest upgrade being able to play almost everything on high today.

I can get the 6870 for $180, but at the same store, for a hundred dollars less, I can get a second 5770 which will give me better performance than a single 6870. Is there any reason(s) why crossfiring is a bad idea?

you often run into driver issues with crossfire and its performance is uneven often giving your very jumpy frame rates as well as microstuddering.

the 5770 crossfire shouldn't be as powerful as a single 6870, pretty even but I guess if you spend a lot less for the same performance its worth it.

I am running crossfire hd-4870's, A6-3650 apu@3.5ghz/550mhz app shader.

It seems to play games well, mostly you have to enable "v-sync" so it runs smooth.
1080p gaming is decent. games like dirt2 run at 43min, 67avg with highest details, 4x af, v-sync off for testing.

If you are running a dual core cpu moving on to crossfire wont help much unless the game supports app opencl then it might use some shaders to help the cpu out.

If you have a tri-core or quad, yes crossfire does help.

My board is a micro atx x16-x4 board. The old x2-4400+ 939 crossfire board was full dual x16 speed slots. I did a few benchmarks. In the ones that test only the gpu power the old board beat the new one by about 5%.

I suspect using larger cards in crossfire with a x4 second slot would make this bottleneck start to matter more. So maybe a pair of hd-6870's might microstudder or something.

In catalyst (vision in win7) you simply check the enable crossfire box and crossfire is good to go.

Make sure you keep the gpu/mem clocks the same on both cards when you overclock them in overdrive. Yes you can even overclock each card when running crossfire.