There are not many 300Mbps cards around yet - D-LINK DWA-556 for example.
Look for ones which have fully detachable antennas and extension cables. It is often the case that the antennas are behind the PC case, under a desk, close to a wall &c and wireless coverage at that location is not the optimum. I have often installed remote antennas which has significantly improved coverage and speed.
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