Check out the Bigfoot Killer-N 1102 ($40, 300mbps, 2 spatial streams) and 1103 ($60, 450mbps, 3 spatial streams) cards (mini PCI-E). Also, check out the Intel Ultimate 5300AGN.
Here's a fantastic review for the
Bigfoot Killer-N vs. the world.
I reckon you're using a laptop? If you're using a desktop, then you should aim for PCI-E cards if you have got a slot and can spare it (do consider it for future use for soundcard, TV tuner, etc...). You can use x1 cards in x16 slots as well, provided that your motherboard can offer 1 lane (probably PCI-E 2.0) to that slot while still offering 16 lanes of PCI-E 2/3.0 to your graphics card (if you have a dedicated card).
I haven't yet seen any WiFi card that can outperform the Bigfoot Killer-N 1102/3. So you might want to look into a mini PCI-E to full PCI-E adapter if you plan on buying it.
