Nvidia's 3D Vision relies on a dual-link DVI connection to pass a 120 Hz signal at 1080p/60 FPS to a certified 3D Vision monitor, and AMD's HD3D employs DisplayPort to pass a 120 Hz 1080p/60 FPS signal. There are not any monitors that can handle both standards (yet), so buyers have to commit to one or the other.
The good news is that monitor pricing is similar, regardless of the direction you go. A 23" 3D monitor sells for about $475 on Newegg right now, whether it's 3D Vision- or HD3D-capable.
Of course, when it comes to 3D, bigger is definitely better. And if you want to get the most out of the technology, a 27" display is an even more interesting proposition.
If you're looking for a 27" 3D Vision-capable solution, Acer offers the HN274H for $680. It comes with a built-in 3D Vision emitter and a single pair of 3D Vision glasses, so a separate 3D Vision kit is not necessary. Because the emitter is integrated into the monitor, this model also works with Radeon HD 5000/6000 cards through the HDMI input. Unfortunately, because HDMI is limited to 24 FPS at 1080p in stereoscopic mode, the Acer screen isn't ideal for gaming on an AMD card.