2.4ghz is the normal wireless band, supported by nearly any wifi card. The 5Ghz channels is supported by most new N cards. 5Ghz is supposed to be faster than 2.4Ghz, but everything i've read says it's not much better.
The main reason to get a 5Ghz wifi IMHO is if you live in a highly populated area with many conflicting 2.4Ghz wifi routers. If all of the 2.4Ghz channels are taken, you can experience a lot of interference, so you get a 5Ghz router to put you on a different band from all the 2.4Ghz routers. If you don't have that issue, there is really no reason to get a dual-band router, your performance will not increase by any substantial amount, especially in your usage situation where your not even pushing 1MBs over your wireless.