N routers offer greater wireless coverage over G routers. that being said, i have a G router that i spent $100 on nearly 5 years ago. i live in a split level home. the router's in the basement and i get a good signal 3 levels up in the attic.
with regards to speed, your ISP will only deliver service AT A FRACTION of what your wireless router is capable of transmitting. if you had a B router (the oldest of the bunch finishing in at 11Mbps) you would STILL have more bandwidth than even the best internet connection can provide (currently verizon FIOS here in the states for residential use).
i would recommend you go with a G router. depending on where all your devices are, it will provide better coverage than a B router (again, my G router covers my ENTIRE HOUSE) at a fraction of the cost of today's latest N routers. i bought a trendnet G router the other day, not because i really needed it, but because IT ONLY COST $10!!! a $10 backup network device is pretty good.
since you can't save much (if any at all) by opting for a B router, you may as well go with the G which offers better technology anyway. however, going w/ a G router WILL save you a minimum of $80 upwards on average.
the fact of the matter is, if you have a good G router, nobody needs N right now. you won't see any real world performance increases in speed unless you're doing HEAVY in home wireless file sharing between devices (which a vast majority of the wireless community doesn't do).
in conclusion, go with the G unless (and this is my final unless) you have to cover a large amount of square footage. if your home spans 100 yards, go with the N. if you're like the rest of us, go with a good G router (my netgear's wireless range pretty much rocks).