Stick with your 5870. Tbh, your platform is going to bottleneck any higher cards or crossfire. I went from a Q9450 at 3.4ghz, which is a comparable cpu to yours and I saw light years of difference when I upgraded platforms to the i5 2500k. Bottom line, you will not utilize a crossfire's/higher card max potential with your current setup so you will see very minimal gains even if you do get another 5870. I wasn't satisfied with my 1st 6870 because I thought it didn't give me the performance I wanted when I was on my Q9450, so I got a 2nd one and gained a full 10 fps, I even upped the OC to 3.6ghz to see if I would gain any noticeable fps gains, I did not.... only then did I realize that I was being bottlenecked by my cpu/platform. I averaged around 45-50fps on Bad Company 2 on my Q9450 with xfire, and when I went to the i5 2500k, it jumped to 120-130 on average, hitting as high as 180-200 fairly often. On the Q9450, gpu usage under msi afterburner would hover around 44% at load for both cards, while on the i5 2500k, both were at 99% the whole time, a clear indication of bottleneck.
If I were you, I would either sell off your system to get a new one, if you really want superior performance. However, your set up is good for any games out now, so that might not be a good option. If you're playing 1080p or below resolution, xfire 6870's would obliterate any game if you do decide to upgrade, and I've seen them go for around 160$ with rebates so xfire would be very affordable.
AMD Radeon HD 6950 & 6970 CrossFire Video Card Reviews - Aliens vs. Predator - Legit Reviews
Starting on page 6, you can see how amazing the 6870 scales vs the 6950 and 6970. It often times come so close to the latter two and that was the reason why I picked it over the premiums of the 69xx cards. To me, that minimal gains (if any in certain games), is not worth the ~70$ premium you would pay for the 6950 and certainly not the 200$ premium for the 6970 and you have to remember that the 6990 is ~10% slower than xfire 6970's because they have to down clock the card a bit to meet TDP. Unless you are playing on a 30'' screen or multiple huge monitors, do not go for the 6970. 6870's are the best bang for your buck.
And PSU - stick with something 750w+ from a reputable brand. I would personally go for 850w+