technically, USB 2.0 wireless N cards should be equal in speed to an internal card. I've never tried it to see, but usb 2.0 is 480mbps and wireless n is 300mbps. Although looking at the ncix site alot of the usb wireless N cards are 150mbps.
This one looks good though...
http://ncix.com/products/?sku=57498&...ture=TP%20Link
for pci-e cards, this one is on sale for almost half price until the 16th...
http://ncix.com/products/?sku=23785&...k&promoid=1283
pricematch for $3 off:
http://www.onhop.ca/product/1178534/...-11n?f=Shopbot
all the cards cheaper than that have poor reviews... unless you have a PCI slot available?