I can empathise with you. I use a Belkin 54g router and extend my network with a Belkin access point. I upgraded to the belkin n modem router and found that the results were terrible. In fact, it had an even worse signal than my 54g router. I tried different settings and called the technical help several times but they were useless. So for now I am sticking to my old setup.
You could try updating the drivers and trying all the channels 1- 11. It's known that 1, 6, and 11 (11 in particular) work the best but try them all. You can change them in the router setup page (192.168.2.1).
I would suggest you are cautious about what you go for next because they don't always do what they say. The n range is supposed to be far superior to g+ mimo, but it's up to you if you want to risk it. Sorry I couldn't give more help.