Mixed 802.11g/b network performance

I understand that b links use DFSS modulation with a theoretical
maximum transmission rate of 11 Mb/s, while 802.11g uses OFDM, with a
maximum rate of 54 Mb/s. I also understand that in a "real" WLAN, the
actual transmission rates are much less due to limitations of the
Ethernet interfaces, etc.

But what about a mixed network? Are the links betweena b/g router and a
g adapter/client PC using OFDM at its higher speed, while a b
adapter/client PC linked to the same router does so with DFSS
transmission at its lower rate?

I recently upgraded my home WLAN PARTIALLY from b to g. I replaced the
Microsoft MN500 base station with a Linksys WRT54G router, and my
laptop D-Link b adapter with a Linksys WPC54G adapter. But my wife's
downstairs computer (which is "off" most of the time) is still equipped
with its old Microsoft MN510 USB adapter.

The router is set to a "Mixed" network. The downstairs computer
connects to the WLAN just fine with its b adapter.

But, even if it is only "on" part of the time, is the downstairs
computer's adapter causing the transmission speeds between my WPC54G
and the WRT54G to be less than they would be in a "pure" 802.11g WLAN?

In other words, would there be any benefit (in terms of transmission
speeds on the overall network) to my upgrading the downstairs PC's
adapter to a WUSB54G, and setting the router to "G-only"?

- Jeff