Yes it was only with Crossfire.
The HD 5970 was a dual GPU.
the 5970 used crossfire, it's 2 5800s on the same PCB
but as far as I know the fixes AMD made for the 7000 series CF frame pacing was also applied to the HD 5000s and higher, only the HD 4000s and lower never got it,
in any case, 1GB in 2016 is hopeless, and now the 6000s and lower are not getting drivers update, crossfire profiles, so yes, it would be a very bad choice.
5850 is pretty slow uses a lot of power and the 1GB limit is a big problem.
you would be better with a 260X or gtx 750 than trying to play games with a 5850 CF
" I am running XP/Ubuntu"
Nvidia is a lot better when it comes to Linux proprietary drivers and they also have better XP support (more recent drivers, more recent products supported)