Hello phil.
Wireless G and N are not too different from each other. While N may claim faster speeds, in reality you won't always be receiving those speeds due to numerous other factors. The good thing about N over G is its farther range. But, in order to make use of the wireless N adapter, you will need an N rated router as well.
Now your router can only put out as much broadband as it is receiving from the modem. And how much as your modem receives is only as much as you pay for through your ISP. And during heavy traffic hourse and dependant upon how the signal reaches your home (the wellness of the cabling, etc) is how much broadband you actually get in real life.
Now if you were getting 25MBps (MBps is very different than Mbps, by the way), that is quite good, and I wonder why you were attempting to get faster? That is blazingly fast.
Now with the dramatic decrease in speed, it could just be the software. Did you uninstall all previous wireless adapter software, and install the newest version of the adapter from its website?