I used to have a pair of 3870s, they were solid. But, AMD doesn't seem to be supporting them anymore with newer drivers, you may be out of luck.
Most people with AMD cards have gotten past the issues due to the updated drivers and/or the +vt_maxPPF 4 (or 8 or 16~up to 256 depending on your CPU power) command set in the launch options.
If they aren't going to release new OpenGL drivers for the older 3xxx series of GPUs, then you probably won't get very good performance in this game, ever.
Maybe you can try getting your hands on the ati openGL DLL file and dropping it in your \Steam\steamapps\common\rage folder. That could screw things up though. Your card might not be compatible with that file.
You can find this file in the temp install folder of the newest drivers. It is called atioglxx.dl_
I'm not promising that this will work, but it's worth a shot.
Now this next part is not directed at you, please don't be offended, It's just a simple observation.
Most of the posters in the Bethsoft forums, and the Steam forums, and all over the internet for that matter seem to be very unskilled when it comes to using PCs. Combine that with their low reading comprehension skills, and the fact that they do not read all the stickied advice threads, or the relevant posts detailing what other users have done to get their game working properly has led to HUGE amount of internet rage.
If most of these people took their ADHD medicine, took a few minutes to read and then tweak their settings, things would work out great for the majority of them.
Hell, I was pissed that I spent $60 dollars to pre-order this game and it was basically unplayable for a few days. But, the gaming community came together, and some of the smarter gamers figured this shit out for us. It was a bunch of trial and error, but after a few days, things are working pretty good.
I think a lot of us are really spoiled these days, we want instant gratification, and if things don't go our way, we throw a hissy-fit.
I'm not accusing anyone here of that, that's just what I've noticed in the last few days of late night troubleshooting for this game.