I have a radeon 5850 in my system (i7 920, 12GB DDR3 1600, SSD, win 7 64). At 1080p I get som stuttering with shadows, reflections and water set to high, but with a combination of low / medium settings i get decent frame rates in 1080p. So I am wondering how much of a difference a GTX 670 will make in this system.
Any upgrade is just that; an upgrade. By switching to a GTX670 you will definatly see an increase in preformance. Just make sure you are not bottlenecking your computer, a great GPU is no good paired with a insufficent ammount of RAM. And make sure you check that your PSU can handle the added power consumption.