Yes your motherboard can crossfire, and cannot SLI as far as I know, when I look on ASUS I don't see your motherboard.. (Intel B43, Intel G43, Intel P45, Intel P43, Intel G41, Intel Q45, Intel G31, but no P35)
I'm not sure my 2004 motherbaord from ASUS is still on their website. I just want to make sure that you have a Crossfire motherboard, and that ASUS has given a BIOS update from 1.0 to 2.1 PCI-e (or if you have a 2.0 slot) on that motherboard.
If all of those are true I would get the card I linked. But, you're going to have to help me out, so could you download CPU-z and print screen the motherboard tab and post it here for us.
I had a p5k back in the day and yes they are crossfire capable but wont do it at x16/x16. I got suckered by this because it says 2 x16 PCIe lanes but as soon as you put in a second card it drops them. Cant remember how much though, might just be to x8/x8 which shouldnt hurt performance noticeably.
I'd probably avoid a 4870cf setup and go for a single gpu setup that has similar performance.