The ATI Radeon 5670 series is the equivalent to the GeForce 250 GTS series.
Another question for you guys.
Regarding SLI/Crossfire, I've always thought of the following as an analogy. Having a video card is like having an actual arm. How strong that arm is is similar to how good your card is. If you're using that arm to arm wrestle someone, you're playing a game or something that requires heavy use of a video card. If you have two arms (SLI/Crossfire), then you can use both arms to arm wrestle against their one. This doesn't mean you're any stronger, it just means that you're aiding one of the arms so it doesn't struggle.
You forgot the part where the arms suddenly disagree due to the brain (drivers) being messed up. And instead of both arms cooperating to beat your foe at arm wrestling you punch yourself in the face.
Wait, why do you need an analogy in the first place?
SLI/CF has come a long way since their debut. And the scaling with 2 cards has improved considerably, you can get up to around 85-90% of raw power increase from that second card (as in, you don't instantly double the performance, but nearly). But there is still issues with some games where the second card just does little to nothing, or even worse slows things down. It's rare though, but you should be aware of it before you buy it.