Increasing Wi-Fi coverage

We are moving our office to a new building and it will be about 5x larger than the one we currently have, so I drasticly need to increase the size of our wifi coverage. The office will be shaped like an "L", so that may influence your advice on this question:

Do I buy a wireless AP, and set it up too so everyone can have fairly good coverage, or do I buy a 7dBi antenna, OR do I buy a wifi AP and 2 7dBi antennas to get total coverage? We might be expanding again later, so I am thinking that I might eventually need both the antennas and the AP, but I am not sure how much access I will get off those 2 things, individually or together. If I do comine the two ideas, will I be lighting up an entire city block with wi-fi coverage, or should I be fine?

Router: DLINK DI-624 (with the ExtremeG)
AP I would buy: http://www.newegg.com/Product/Product.asp?Item=N82E16833127138
Antenna I would buy: http://www.dlink.com/products/?sec=3&pid=416#

Thanks in advance guys! :D

You should be fine with two standard wireless access points. Put one at one end of the office and one at the other. Don't they have a range of upto 100meters?

Yeah they are supposed to be about 100m range, but that will get you like 1% signal length. My boss is currently unsatisfied with his 50% connection, even though he is shooting the signal through a big metal cabinet. :mad:

Yeah, I told him to not do that when we move, but his ego gets in the ways of his ears sometimes, and now he just wants me to make sure everyone gets a high signal, or else its my fault that they don't know how to arrange furniture. This guy is set on spending some money, and I am just not sure which method listed in my first post will gurantee me from getting yelled at for less than perfect connection rates.

We just changed a clients setup (in a warehouse) to use the rangemax gear ...

http://www.netgear.com/products/details/WPN824.php

The difference was immediately obvious , but since to get the full benefit we changed all the cards to rangemax versions as well then an access point or 2 would be the easiest/cheapest ?