Most GoPro cameras can film in 4K (3840 x 2160), but older models are limited to standard HD (1920 x 1080). Premium models can film in 4K at higher frame rates like 60 frames per second (fps), which results in smoother, more realistic video. If you need superior 4K video quality, you’ll need to look at the more expensive GoPro cameras, but if you need great but not amazing video, you can save a decent amount of money by getting a prior-generation model that records in HD.
Although obvious, I put this here first because this is one of the most important factors when purchasing your camera. The HERO3 has three editions (white, silver and black), ranging from $199 - $399 (prices continue to drop annually as newer versions are released -- we'll keep you updated on those). The HERO2's are at the bottom of this gap, but aren't as "new" and lack some features the 3's have (they're discontinued at this point but can be found on the net here and there).