I don't think there is 802.11g available @ 5Ghz, only 'n'.
I have a new apple airport extreme base station, it has backwards compatibility mode so it can talk all 2.4ghz wirless cards b,g, and n. You can also lock out one of the configurations so its only b, g, or n at 2.4ghz or only 5ghz n.
Since I had a B router then upgraded to a G router , then recently got the airport for n compatibility, I locked each router into its own version, thus my two linky's were b and g only, and my APE basestation to 5ghz N only.
After retiring my last PC and the related b airport cards since they only supported WEP, I only have two routers. The drawback for being backwards compatible, you lose the higher speed capability of an N only 5ghz wirless network. since I have a family of N compatible iMac's and laptops and a couple g only Mac's, they communicate to their respective routers, so I can run N at full speed.
I'd recommend reading up on the APE :
»www.apple.com/airportextreme/spe ··· ecs.html
for an idea of its operation. But it will communicate to both your laptops with no problems at all in compatibility mode. Since you have a macbook already it makes short work of configuring the APE.--
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