Using Macs and PCs here, so would prefer something that ran natively on each. (Yes I know Macs run BSD UNIX, so anything can be done).
I took your advice and tried this. I swapped the 100mbps switch I had my PS3 and Tivo HD plugged into with an old G router with a 100mbps switch, and am using the G router for my son's old 802.11b iBook (WPA2 killed it; but back online using WEP) and the 2nd gen iPod Touch (using g). The G router has DHCP and Firewall off…just a dumb switch/Access Point. I put the G on radio channel 1, and put the RT N16 on Radio Channel 6, with 40Mhz Lower. Should be no overlap of channels. The Asus is N only now.
I did the same file copy and guess what? NO DIFFERENCE in speed. Same as when I was doing 'Auto' (sharing with G devices using WPA2/AES only). Crap. I did compare it to the old G router which predictably took over twice as long as the Asus.
While I cannot figure out any difference in speed using N only mode in transfers, I will say that subjectively my ThinkPad W510 running XP *feels* faster. It felt sluggish before surfing the internet, which is odd given that even G is oodles faster than my WAN (8 gig download). Feels better now. Could it be the radio on my Thinkpad likes talking better to an N-only router? It is odd.