If it will, do you think I should buy a second 6870 and crossfire them, or perhaps purchase a GTX with 2GB of VRAM. (Which one would be the best performance upgrade for around $250?
And my final question, are there going to be any new graphics cards that would impact the current performance in the $150-$300 price range? If so, should I wait and purchase the new model, or purchase an older model that will be lowered in price?
In the next month or so the AMD Radeon HD7950 should be coming out (Possibly along with the 7870 and 7850). This card should be priced around the $350 mark (with the 7800 series ~$100 less) and will be significantly faster than the current 6870 or GTX560.
I would wait for the 7950 or 7870 to replace your current GPU. You can use your 6870 as is in your new build for the moment, the only thing I would recommend is getting a PCIe 3.0 capable motherboard (and CPU if Ivy Bridge is out by the time you are buying) to ensure best performance out of your new GPU when you get it. (Please note that PCIe 2.0 has enough bandwidth for the 7970 ATM, this would be forward thinking for future GPU upgrades).
It is better to buy second 6870 for many resons. First one is the new 7xxx series Radeons will be expensive. Second reason is that you will always lose money on reselling your 6870. 2 x6870 will be enough to play in full HD for next 2 years. That what LegendKiller is writing is true but 2*6870 will approximately use a little more than single GTX580. PCIe 2.0 is enough for next 2 generations of cards and in computers there is no such thing like "future thinking" because there might come a new solution, kind of port which will cause that PCIe will totaly die like AGP did. Don't buy anything for the future, buy it for now and for your present needs.