The only internal mini PCI card ones with N that I ever found were these from Sparklan and TP-Link:
I don't have one so can't tell if they would work with Windows Vista.
I tried an USB stick but it's a bit weird to have this installed all the time. What worked well was a US Robotics PC Card. It's this model, worked perfectly fine:
US Robotics Wireless 802.11n Ndx Laptop PC Card