You could probably fit a Radeon 4870 and a Phenom II X4 into that extra $150. I would certainly try. Between the two, I would go for a better video card first.

In terms of performance (average): GeForce GTX 285 > Radeon 4850 X2 > Radeon 4890 > GeForce GTX 280 = Radeon 4870 > GeForce GTX 260 > Radeon 4850