Upgrading from 4870's Xfire

a 550 is approximately as powerful as one of your 4870s... so definitely stay away from that card. I would say go for a 570 at minimum if you want a nice upgrade from what you have.

Toms actually puts a 4870x2 on the same level as a gtx 570 and above a 560ti.
( http://www.tomshardware.com/reviews/gaming-graphics-car... )

If you're on a budget you could get the 560ti and overclock it pretty nicely as well, but I am not sure if it is sufficient for 3d gaming as it is quite demanding I hear

Yeah your last statement is probably my biggest issue. I havn't heard much about 3d gaming requirements. And another thing I want to get away from is the major heat issue I had with my 4870's. I was hitting 110C easy.

The HD 4000 series was infamous for running extremely hot. Current cards from AMD and nvidia run a good deal cooler than your 4870s, with temps usually capping out in the 70s or 80s if you aren't overclocking your cards.

3D gaming requirements are high because to make the Stereo 3D effect work at 60FPS, your GPU has to be able to pump out 120FPS consistently, 60 FPS for each eye. You also need to get a new display that supports stereo 3D if you don't already have one. If you want to play on high settings at 1080p with 3D, you're probably going to have to look at the extreme high end single GPU cards like the GTX 580, or look at an SLI setup, like 2 560Tis for optimal performance.

Hm, I think I'll buy a 560ti and go with a lower resolution/graphics setting to save on money.

I say go for at least a 570. A 560Ti is only going to be about as powerful as what you have. Of course, nVidia will be releasing new cards in March so just keep that in mind