2.4 GHz is the most common frequency. The only way you can harm the antenna is to load it with extremely high transmit power, but you'll probably burn your radio chip inside the WiFi card before you do damage to antennas. Conductor makes a huge difference in range and speed and not as much as frequency.
Here's how MIMO works: How Stuff Works – 802.11n MIMO | WiFi Jedi.com
BTW, it all depends on radio chip if it supports 2.4 and 5 GHz and not the antenna specs. It's actually the antenna GAIN that you're supposed to look for and how much antennas needed by the WiFi chip to perform to its optimum speed.