With the 4870 being so old and slow compared to the newer stuff, you'd be better off just replacing the video card. I'd highly recommend the cheap 6850/6870 if you are around the $120-150 price point; that should net you close to double the performance. It gives you dx11, runs faster, cooler and uses less power.
As for the difference between dx10 and 11? There's a few things they added, but the biggest improvement (and its actually marginal in comparison) is that the performance doesn't suck as bad as dx10 did. By the time game developers were about to code using the dx 10 api's, 11 was right around the corner, so a number of developers just waited for 11; you'll hardly see full fledged dx 10 apps. Even today, its mostly just between dx 9 and 11, with 9 being used for older hardware and consoles.
but which would give you more (I guess you could say in general?) SLI or Xfire?
Honestly, both multi-card solutions are very refined now days; it used to be SLI was way better and more stable, but xfire has proved itself to be a good contender since the 4k series. It really comes down to card vs. card performance and how well the games are designed to take advantage of gpu scaling. It can easily go back and forth which is the better platform. But I've built newer xfire systems that didn't have any driver/xfire issues at all. So you win some, you loose some when you do a DIY build.
Personally, I think SLI is better with scaling than xfire, plus nvidia's drivers are just much more solid than AMD's in my experience. I just hated the fact when I was running my 5k series that the only stable gpu driver set was from august 2010, and I only upgraded back to nvidia 5 months ago.