First of all: 3d gaming is pretty awesome, I have an S23A750D and it works great, however the newer S23A950D seems to be easier to get these days(it may be cheaper than the 750). You can try it out 3d before you buy your video card (it will work in some form with any hardware, 3d takes alot of horse power, so if you want to go 3d, keep that in mind).
AMD cards started supporting HD3d (AMD's version of 3d vision) in the HD 5xxx editions I believe, if you have that version of card you can run 3d in frame sequential mode. If your card is older, you'll have to run in side by side mode, where the each perspective of the game (left eye and right eye) are rendered at half the width of your screen, and your monitor gets set to seperate the two images.
If you enjoy 3d, keep in mind that to get 60FPS/eye you have to render at 120FPS, and there is some additional overhead (not sure how much) from the 3d rendering driver (Tridef or iz3d).
Any video card will 'work' with 120hz mode, however there are some things to be aware of:
120FPS is a very high bar to hit with PC performance without sacrificing settings. I use an HD 7870 in 3d mode, and typically get 45FPS/eye in skyrim on high. (I wish I had gone a little higher on the video card, but oh well

)
HDMI is not capable of 1080p@120HZ (it's bandwidth limited to 48FPS@1080p). Display port and Dual-Link DVI are the only digital connection methods that do (I think VGA also does I think, but not digital).
I have heard that DL-DVI is not supported for HD3d, but I can't confirm it.
If you get the display port monitor, keep in mind most 7xxx cards are mini display port, buy a cable that is display port at one end and mini DP on the other (or be sure your video card comes with an adapter, mine didn't cost me a week of waiting).
edit: I would also argue that there isn't any perceivable difference between 60hz and 120hz gaming, the eye/brain combo we all use does a pretty good job of blending frames together, which is why 60hz has been the standard for such a long time. IMO 120hz is a marketing gimmick (especially in TVs), however there really isn't much data on either side of the argument because it's all subjective/relative, and you will always have people who can 'tell' the difference even if there is strong proof that it doesn't matter (see "over sampling" in audio). _end_rant_