i got myself a USB Linksys EA3000 for one of my pc's and a extreme N PCI-e card for my other pc . The USB EA3000 has 3x3 antennas so its actually more performant than the PCI card. it DEVOURS the PCI card. It has the highest connection rate found in N cards. 450mbps. 99% of n adapters are 300mbps.
In any case, both PC's are side by side, about 50feet from the router,the USB wins every time with incresibly low ping and a full 5 bars all the time, as opposed to the PCI card that has 3 external antennas that gets 3 bars all the time. At the moment the ea3000 is the best... well aside from the monster ASUS (PCE-AC66) , also that thing is beautiful.