Wireless Adaptor causing poor broadband speeds?

[#1]

I have been with Plus Net since the start of the year - on normal ADSL I was getting 7-8 MB consistently which was in keeping with what I was expected and received on my line previously. 
I upgraded to Fibre Optic a few weeks back and am expecting in the region of 27-30MB.
I am getting this when connecting via Ethernet but when connecting wirelessly (which unfortunately I need to given the location of my home PC), the speeds are fluctuating massively.
When I first connected, I received 25MB but since then, I am averaging about 15-17MB, and sometimes as low as 8-10MB.
To be fair to Plus Net, they've carried out all kinds of checks and as the line speed is itself is ok, there doesn't seem a lot more they could do.
I therefore thought it must be something to do with the wireless router I've been sent (Netgear N150), i.e. either it's faulty or it's an entry level model that Plus Net send out.
However, it's been suggested tonight that I should check the wireless adaptor that I'm using, to make sure that it's on the correct B / N / G setting. 
I'm not really sure how to check this but looking in 'Device Manager', my Network Adaptor is showing as 'Broadcom 802.11g'.
I can't see any way to change this so am assuming this may be what is restricting my network speed?
If so, what options do I have - is it possible to change my Network Adaptor from a 'g' to a 'n' or do I need to buy a new internal Network Card or USB Network 'n' adaptor?
Any advice gratefully received - in simple terms please
Many Thanks
Pete

[#2]

If device manager identifies the adaptor as 802.11g then it's almost certainly limited to 'g' speeds which could well be limiting your speeds to the laptop.
On some laptops it's quite easy to change the WiFi card but it's definitely a lot easier just to plug in a USB device, a reasonable example of a low cost but fast 'n' model is this one: http://www.pc-titan.com/product_info.php?currency=GBP&info=p53329_Getnet-GN-621U--300Mbps--USB-2-0.h..., I have one of these on a desktop and performance has been very good for such a low price.

[#3]

16->17Mbs is pretty much the most you will get on a wireless g interface so it is that that is limiting your speeds. Unless the device has an option to set wireless 'n', the only way to achieve a faster speed is to change the wireless card. It's difficult to get internal upgrades so an external USB wireless 'n' dongle should be OK, as long as you have USB2. Many 150 n dongles about, and most will work fine as long as drivers are available for your OS. I temd to use ones that have a flying lead so I can move the dongle about to get the best signal.