Do you get the 50Mb wired? If so Id check the following...
router away from any sources of interference i.e TV/power cables,not near fish tank etc
make sure the only devices connected are "n" if you have four N devices but another at G the router will only work at G speeds I believe. Set the router to "N" only if possible
To check the best channel download something like this
http://www.metageek.net/products/inssider to see if you are clashing with something on the same channel
Hope that helps