the 48x0 and 38x0 series have a huge advantage: any two of them (even the X2 models) from any manufacturer can be used together in crossfire. you can still cf the 3670 and 4670, but there's only 1 type in each series. right now, the 48x0 series is expensive, while the 38x0 series is getting cheaper every week.
if you want a cheap, good card, the best choice is the 4670. if you want the best card, it's the 4870 X2. if you want the best price/value, i would go with the 4850.