To make it simple.
54mbps is the radio link to and from the router and your computer. In a perfect world you might hit that. Most likely not.
100mbps is the hard wire connection to and from the router and your computer. Some things can affect that speed also.
Those 2 figures are top speed ratings. If you scope out your connection info in your operating system you'll see it shows top rated speed, not what you're actually running at.
How fast the WAN port goes is up to how fast the connection goes. And even there it's a crap shoot. There's a lot of variables that affect the actual speed. Like signal compression.
You pay for the bandwidth but most likely don't get the full amount. I'm on 18 down and 2 up. I get 17.26 down and 1.47 up.
As for directional, as in aiming the signal, TP-Link makes all kinds of antennas for reasonable prices. But the drawback to aiming is anything in the path of that aimed signal past you is going to get the signal also. If you go that aiming route, put the directional antenna on the computer and aim it at the router.
If you can hardwire in, you'll go a lot faster than you do over the wireless. You might even want to look in to getting hardwire over AC lines as an alternative to running a cable.
---------- Post added at 11:18 PM ---------- Previous post was at 11:14 PM ----------
Also microwaves and some cordless phone can toss up interference to 2.4ghz signals. And if they're in the path between you and the router it can block the signal while they're in use.