What speed is showing in task manager, and what speed are you getting across the network?

I've just managed to sort mine out so I'm now getting 300Mbps @ ~16MB/S using an Intel 5300 and Airport Extreme

If you have the airport extreme DON'T upgrade to 7.5.2 use 7.4.2, if you have already upgraded, then down grade, as 7.5.2 screwed mine! resulting in half speed!

One thing to note about wireless N is that while it is faster and also covers a better distance it's penetration is worse.
I have noticed through lots of walls in the house I am in now, even though they are just wood etc you get what your seeing there.
Be in the same open room - dining room, kitchen, living room we have and I know even if I was further away it is better.

I bought an extender to get it running better in my room on the other side of the house.

Yep not a problem for me as I have two APs in the living room (2.4Ghz G and 5Ghz N) and another over the opposite side of the house so get good coverage anyway, mainly when using the N I'm no more than 5m away, as the main driver is to be able to transfer large amounts of data across the network quickly, so I don't have to plug in the cable

Wireless N penetration is only worse in the 5GHz band. If you use 2.4GHz Wireless N it should have the same penetration as G.