Good call asking all this before you put the cash down.
The game is under some heavy revision currently, with a new graphics engine being developed that is supposed to improve performance, so it would be a good idea to wait for the next patch before making any significant upgrades.
That being said, here's how things are for the time being:
1) The old conf.ini file had a lot of leftover settings from the pre-release/development/testing phase in it, when the developers needed them to set certain flags to different values and check how things work.
A lot of these settings were obsolete and also posed a danger of causing problems for the game if changed by the player. So, a few months ago in one of the patches they streamlined the .ini files by issuing new ones that had these setting removed. The affinity mask was one of them.
2) If you can wait a while before buying a new graphics card, then wait. The 5870 is the previous generation of DX11 cards by Ati, the 6870 is the current one. I don't really know which one is faster spec for spec, but what i do know is that the game likes a lot of VRAM in its current state. In that sense, buying a card with more than 1GB is best.
But like i said before, the graphics engine is being updated in the coming weeks, so if you can still fly with your 5870 stick with it for a while longer: if the new engine improves performance for you, you might not need a new GPU after all and you'll save some money.
3) The worst thing you can do is go for a dual card setup. Both Ati's crossfire and nVidia's SLI dual-GPU configurations are not working well enough. In fact, some people get better performance running the sim on just one of their two cards by disabling the other one.
Also, it won't help with VRAM, if you have two cards with 1GB each the total you'll see is still 1GB. I think this has to do with the fact that both cards need the set of the frames to be rendered in their memory (most of the times card A does frame 1, card B does frame 2 and so on, but each card keeps the complete set of data in their VRAM), so while you have double the VRAM you also load the same information twice (once for each card) and you end up back on square one.
Of course, that could still change due to the upcoming patch and maybe dual GPU setups will be better implemented.
Long story short, stick with your 5870 for a while more but if you really, absolutely, have to upgrade then get a single GPU card with more than 1GB of VRAM.
