I already have a 3870 and with the price drops I can get another one for $129 and free shipping... I have $120 sitting in my paypal and its all I got to spend on computer stuff ( I can come up with the extra $10....)
But if I go 4850, I gotta come up with an extra $90 or so, and then sell either the 3870 or my wifes x1950pro.....
And the 4870's are just way to out of my price range.....
If I remember correctly, crossfire doesn't scale correctly in most games, especially if you don't have a X38 or X48 motherboard. As the 4850 is better than the 3870x2 in most games, it's probably better than two 3870s in crossfire.
You can get a 4850 for 150$ if you find the right website, or 175$ at most retailers...so just sell you old 3870 and get a 4850 with the extra 60$.
Edit: Didn't see you had a X38 mobo. You should find some benchies on how two 3870s in xfire perform against a 3870x2...and then compare the performance of the 3870x2 vs. the 4850. A simple rule of 3 should give you your answer
You can CFx a 3850 with a 3870, they are close enough that the 3870 can scale down to a 3850. the 4xxx series are a completely different beast. I doubt the 4xxx will CFx with a 3xxx series card, but the 4870 *should* CFx with a 4850. Again, only clock speeds separate these cards, so they should be able to CFx with each other.
As for what you should do, I'd probably get the 4850. CF performance varies to much, the 4850 provides constant performance. Sell your x1950pro. Add that to your Paypal account, hopefully that will be enough. give the 3870 to the wife, and enjoy the 4850. I'm not sure you can sell a x1950pro for $90, but do your best.