The 802.11n specification defines the parameters for speeds up to 600Mbps, but most parts of the 11n specification are optional. I think wireless routers and adapters that support 600Mbps are pretty much non-existent (600Mbps on a single band, adding 300 on 2.4 and 300 on 5 doesn't really count).
The mandatory parts of the 11n specification require up to 130Mbps for a wifi access point, and up to 65Mbps for client devices (computers etc.). Apparently the Dell Wireless 1506 card in the Dell 660S is only 150Mbps, and would likely only operate at a max of 65 or 72 in the presence of neighbouring wifi networks. And then the speed that you can transfer data over the wireless is also less than those numbers.
Those tests in an unrealistically ideal environment showed that the 130Mbps Plusnet TG582n router could just about cope with the 77Mbps of the fastest possible Fibre broadband.
All parts of the wireless network do not need to be 11n to see any benefit. There is no requirement for everything to transmit everything at the lowest common speed, there's no reason to, most data is transmitted from the router to one particular device, it doesn't matter if other devices can't / don't receive it.
No there probably isn't much point in getting an 11n router. A cheap gigabit ethernet chip probably doesn't cost much more than a chip that only does 10/100 ethernet. Presumably a 300Mbps (2 antenna) or more 11n wifi card does cost slightly more than a only 150Mbps (1 antenna) 11n card.
