A GPU equiv to ati 5970

Nope, there is none, it is the fastest card there is. The one that comes closest is the GTX 480, but it is still quite a bit slower:

http://www.tomshardware.com/reviews/geforce-gtx-480,258...

really, another silly question, when using dual gpu, do they have to be the same
i.e 5970 x2 or can it be 5970 and a 5870?

You can crossfire a 5850, 5870, and 5970 in any combination. However the crossfire setup will run at the speed of the slowest card.

You can either overclock to match the speed of the higher cards or stick with lower clocks.

Yes, but only if the two cards use the same core (for example 5870 and 5850) then the faster of the two will clock down to the speed of the slower.

However, the 5970 is a special case since it is a dual GPU card (2 slightly downclocked 5870s on the same PCB), therefore, if you add another 5870, it will clock down slightly to match the two GPUs on the 5970 and you will get tri-fire.