True, but if you crossfire 2 4780s, you got a decend card for the money
I'd still rather get and crossfire two GTX 460s or a single 5850 (which is comparable to crossfired 5770's).
2 4870's outperform a 5850, however you don't have DX11, you use more power, produce more heat, not all games utilise multi-card systems and there are potentially more issues that can arise with multiple card setups
With $600:
CPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103882 $99
PSU + Video Card:
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.514501 $260
Case + Memory:
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.514344 $113
Motherboard:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157200 $85
$557 before MIR
$517 after MIR
Only problem you may encounter with that is with windows, and the installation, and then authentication of. If it causes a problem, you will want to buy a copy of windows, which is $100, so would put you $17 over your $600 budget:
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116754
You can save some by swapping the Video card + PSU combo to a 5770 instead:
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.514493
That will save you $70 before MIR, $65 after MIR, but a GTX 460 is much more powerful (To Zua, that is the best bang for buck right now

), so it may be better to get that than the 5770 even though it means spending more. A 5770 will still play anything (speaking as someone with one who plays on full settings on everything I've tried so far, which include your latest DX11 games) you throw at it, but not at high resolutions too well, or as games improve over the next few months/years at full settings, where as the 460 will still be plowing through games without a problem for quite a while to come, as it is nearly equal to a 5850, but a HD5850 pulls ahead in just about all scenarios