I suppose its up to you whether you want to upgrade now or not. People will always say wait for this and wait for that and in that time you can be enjoying something and chances are will be for sometime after that. A 670 will last you quite a while and with how pricing has been its been getting cheaper and will continue to getting closer to the release of the new cards.
I wouldn't go with AMD If you have 3D in mind NV handles 3D a bit better. Also, in terms of memory I would go with 2 gigabytes unless you were running a display that was above 1080p but to my recollection the TN+ panels that are 120 hz do not exceed 1080p so you are safe.
When you go with a SLi system I would recommend a 750 or 850 watt psu. Sure you won't need all that power. You will however have a little extra and its not good on a power supply unit to put all the stress of putting it under full load anyway.
thanks for the help, just out of curiosity, staying at a 1920x1080 without 3d woulld the xfx dd 7970 ghz be more powerful than the 670 ftw on average? also which of the cards runs cooler, my guess would be the xfx correct?
It's a different story when it comes to 3D because Nvidia side stand on top with 3D awhile AMD are below with 2D... A 670 can be OC to be as or faster than 680 right out of the box but 7970 is still a bit faster especially when it's OC...