Thanks for the answer.
I've a fear of wasting money^^
And if the 280X downclocks itself because the 7950 is too slow for it, it's a waste of money. I found the following answer from a tomshardware-user:
"You can but it will slow the other card down. The R9-280X is the HD 7970. When the R9-280 is released, it will be the same as the HD 7950. I would wait for that one because it will be cheaper and will give you the same power. Having said that, the HD 7950 can currently be had for $200 so you might just crossfire that."
Does the 7950 slow the 280x down even if it is overclocked? Means: Does the 280x just uses (per example) as much shader units as the 7950 got?
I just want to be sure, that i understood how it slows down. It would be *****if both card exatly render 50% of the final picture. Even if the 280x sleeps while rendering while the 7950 is working as much as a indian childworker (sorry for the offensive joke).
I'd appreciate it if someone would upload a benchmark or something