The HD5K series will be best for longer terms, and TWIMTBP games only matter when they try to pull shenanigans, and then the community figures out how to work around it, and then they go to the box for 2 minutes to feel shame. You might see slight differences, but they are not major, and the amount of difference doesn't negate the performance differences of the better hardware.
Right now, there's nothing you can buy from nVidia that will last as long, and only the GTX295 is in the same league as the HD5870 with the GTX285 being in the HD5850's league, when the HD5970 comes out it will be in a league of its own, but ALL of those require a new PSU for sure, and you may want tweak your CPU to get the most out of them.