Is your 4870 struggling with what you currently play? If not I would go with option D) Do nothing. Then when it starts to struggle hopefully GPU prices have dropped a bit
Direct x 11 is not a big deal yet, so again I would not bother buying a new card if your current one does fine
If your 4870 is struggling (I would be surprised if it was, my 8800GTX was still running things fine on high quality), personally I would grab the 5870, you have a very nice system, having old technology for your GPU seems a shame.
There is absolutely no need to buy 2 5870s or a 5970, possibly in a few years but not now.