Well the 670 is a hell better than the two AMD ones. You will have to take into consideration the use that you re gonna make of your gpu, then decide upon your needs. Now comparing the 7870 with the 7850 the latter is definitely a better choice since the 7870 is overpriced without offering more than the 7850.
Reviews are saying that the 660TI will not be much cheaper than the 670 (probably around 50 bucks or so) but the problem is that the 670 is not much better than the upcoming 660! The only difference is the Memory Bus (192-bit for the 660 and 256-bit for the 670). Also the 660 will use less power. Moreover the 660 is better than the 7870 and the 7850. If you need my opinion wait until the 660 comes out. It will be the most valuable choice for you and you will save a few money for your SSD.