Thanks for the reply.
I also did some additional research since you weren't completely sure.
Different manufacturers are indeed fine and different cards works if they have the same GPU:
http://sites.amd.com/PublishingImag.../WebBannerJPEG/AMD_CrossfireX_Chart_1618W.jpg
In my case a 7870 with a 7850 would work. Downside: It doesn't make any sense because the 7870 would be under-clocked automatically until it runs with the same clocks as the 7850.
The same would be the case with two 7870 cards. The one with the higher clock runs at the clock of the slower one.
My mainboard supports Crossfire.
Funny story: My flat mate bought a second Geforce and a new PSU (which he didn't need because his 650W PSU was just fine for the SLI system he wanted to run) a few months back only to realise that his mobo only supports crossfire, not SLI. I was almost pissing myself because he bought a new card and PSU for a total of €250 and couldn't use it.
My PSU should just be able to carry 2 x 7870 with a bit of power to spare (both in terms of total Watts and amps on the 12V rail).
But I'm not so sure if it's actually worth it to buy a 2nd card because my 7870 can still run most games in 1920x1080 and good settings without dropping below 30fps.
Maybe it would be smarter to save a bit more money and go for an entirely new card next year.