The data rates for 802.11a and 802.l1g that are higher than 54 Mbs are achieved by manufacturer proprietary schemes. To take advantage of the proprietary schemes, both the Access Point and Client hardware will have to be from the same manufacturer and capable of their manufacturer-specific "turbo" scheme. Some manufacturers claim upwards of 72 Mbs, while others (D-Link) claim upwards of 108 Mbs. I'm using D-Link stuff that claims 108 Mbs for both 802.11a and 802.11g. Of course, the actual throughput is no where near 108 Mbs. These claims are based on the max rate possible (in theory) in RF clean lab conditions.
Even though the "turbo" doesn't come close to what it's claimed, I've found that it does give about 30 percent boost over vanilla 802.11a with my D-Link stuff, which I'm happy with. Can't say about 802.11g as I don't have any client hardware 802.11g.
Keep in mind that the proprietary "turbo" schemes, while giving some modest "boost", will also introduce new limitations. With "turbo" enabled, your wireless range is somewhat reduced. Can't speak for the other manufacturers, but with D-Link using "turbo" requires that your center frequency for "turbo" to be on channel 6 -- which may be more prone to interference. Also, (at least with D-Link), you have two options for "turbo" that are either dynamic or static. With "dynamic turbo", if you have a "turbo enabled" connection established and another connection (non-turbo) is established, then everything will drop back to vanilla
802.11a/g. With "static turbo", only hardware capable of "turbo enhanced" connections will be permitted to connect.
Cheers,
-ES