802.11g adapters usually perform better than older 802.11b adapters, but not always, and it's hard to make more general statements than that. A laptop with a built-in adapter and an antenna in the lid should perform better than a plug-in card with the antenna in the card. "Perform better" might be defined as a higher data transfer rate. Or it might be defined as the ability to connect when the signal from the access point is weak.

But in order to really compare two installations, you have to have the same software. When Linksys software shows an 80% signal strength (whatever that means) from a Linksys card, that might or might not be a better connection than a 60% signal strength shown by Belkin software from a Belkin card.

I've got two Thinkpads, a two-year old X40 with an Atheros WiFi card built in, and a new X60 with an Intel card built in. With the computers side by side, and using the same IBM/Lenovo software, the Atheros card can detect and connect to distant access points that are too weak for the Intel card to even see. But in a strong signal environment, they both have the same data transfer rate.