You do know you can't do that now with a HD 4 series and a HD 3 series. Only within the same series will you see a benifit. As well, if set up a "Mixed Crossfire" setup it just underclocks the faster card to the slower cards clocks right now. Ex: 4850 + 4870
The 4870 is clocked at 750Mhz core clock speed and a (900Mhz multiplied by the quad data rate of GDDR5 ram=3.6Ghz) 3.6Ghz Effictive Memory clock.
The 4850 is clocked at 650Mhz core clock speed and a slower GDDR3 Memory at (700Mhz multiplied by the double data rate of GDDR3 ram=2.1Ghz) 2.1Ghz Effective Memory clock.
Put them together and you get basically 2 4850's.
Here's a review of it:
And to answer your question newbie1323, people are recommending ATI cards and crossfire motherboards because there simply better than Nvidia with Price to Performance. When ATI released its 4xxx series ATI became the Video card company to buy from. My 4870 plays some games better than a GTX 280 which is $400 or more. And it plays most games better than a GTX 260.
If I were you, I would wait till next year. AMD is releasing the new 45nm Deneb cores and in Q1 2009 (For AM2+) it should be releasing the new AM3 socket (Woot, DDR3 support!) as well as Deneb Quad cores for the AM3 socket in Q2 2009.