I thought the p45 was 16x/4x in crossfire.quotemsg]
No, that would be the older P35 that offered PCIe 1.1 16x 4x CrossFire.
The P45 has 16 PCIe 2.0 lanes that can be used for either a single card or split 8x 8x for CrossFire. Additionally, some P45 based motherboards, like the P5Q-E, offer an additional 4x PCIe 1.1 slot that can be used for a a third card.
As far as 4870 1Gb vs 4850 Cf....
The cheapest
4870 1Gb is currently $269.99 on newegg.
As you can get a pair of
4850's for $279.98 and get increased performance now, it seems to be a better option.