7970 crossfire with 5870

You're asking if you can crossfire a 7970 with a 5870?

no i know i can do that I'm wondering if my psu would could with those two in crossfire

Who told you you'd be able to do this? That information is false, though if it were possible, then a 750w psu would be able to do it.
For further information, only 2 gpus with the same first 2 numbers (ex: 79xx) can be crossfired together, though with the micro-stuttering it would create, it is not recommended.

ah well i guess it isn't worth upgrading i don't think the fps gains i would get are worth it

thats what I'm saying if i cannot crossfire them the 7970 doesnt really improve fps much over my current setup not enough for me to spend £400

haha ya u cant crossfire those together

I see this question a few times a week. Why is it that people start asking questions without doing even the slightest amount of research into what it is they're asking about?