Which to keep

I don't know where you are seeing that but the HD4870x2 should be the faster of those two setups. However the crossfired HD5750 will run much cooler and use much less power while giving you DX11 compatibility.
Personally I would sell both and just get an HD5850 and OC the hell out of it.

I'm really not interested in OC'ing my cards, as I would rather they last a significant amount of time.

If you could point me in the direction of the information about the 4870x2 that you are seeing. From where I sit, it looks like the 4870 is on the losing end of a 5750 Crossfire setup, not even counting the DX11 support and the Eyefinity.

OCing does not shorten the life span of anything unless you are pushing it to the limits into temperatures that are unwise.
As for the relative speeds it's the HD5770 that is on par with the HD4870(although the 128-bit bus does limit it slightly in crossfire) not the HD5750. The HD4870x2 is two HD4870s so it will be faster.
Basically it would go in this order in terms of average frame rates;
HD4870x2 > CF HD5770s > HD5870 > CF HD5750s > HD5850

Where are you getting this information? I am looking at the Frame rates for Quarter 1 of 2010 on the forums here, and I'm not seeing the 4870X2 pop up anywhere near the top.

I wouldn't use the Tom's charts for anything basically. They aren't reliable. Look for real reviews.

Ok, perhaps you could show me where this information you have is from. I am honestly interested in finding these answers and my information is just not linking up with what you have provided.

It's hard to find reviews directly comparing those cards but most HD5870 reviews include the HD4870x2 and show it being slightly faster on average. Most reviews with crossfired HD5750s also include the HD5870 and show it being slightly faster.

http://www.techarp.com/showarticle.aspx?artno=88&pgno=4

Just go to this site and compare between 4870X2 and 5750 yourself.
@ Arkaon

Mubin, I appreciate the help and the chart. The info you gave me was good and It would seem that pure hardware wise the 4870X2 will outperform the two 5750's.

The next question, is it worth it to keep the 5750's for DX11?

I probably would. HD5750s should still perform very well and the power usage, lack of heat and DX11 are worth a lil less performance. You should be able to sell the HD4870x2 for a nice sum especially if the send you a new one.

I agree with Jyjjy. In near future DX11 will require most and two 5750 will be very good.