The Dell 1703 Wireless B/G/N adapter is based on the Atheros AR9485 - which is a single stream chipset...
http://www.wikidevi.com/wiki/Dell_Wi...11n_%2B_BT_4.0
In a 20MHz 802.11n 2.4GHz channel, 72Mbps is as fast as your going to go with that particular adapter - you might get a bit more by using wide channels in 2.4GHz, but friends don't let friends do wide channels in the ISM band if you want to be a good neighbour...
Welcome to budget laptops
One option for the 14z is to replace that card with Intel Centrino Wireless-N 2230, as this is a configure to order option for that model.
enjoy!