I have a Sony VAIO laptop with an Intel PRO wireless 2200BG connector networked using a Linksys Wireless G Broadband router with Speed Booster.

My Windows XP software tells me my connection speed is 54 Mbps but I don't enjoy that speed. Linksys says I won't get maximum speed and range if I don't have a matched Linksys adapter for the router.

Is this true, or marketing hype?

You must bear in mind that 54Mbps is the thoretical maximum for this connection.
Once you factor in overheads etc it drops down.
Admittedly the Intel chipset isn't the best on the market, but I'd stick with it.
Otherwise you will have to spend out good money on a linksys pcmcia card that may only give a small overall gain in performance.

What may work better is resiting your router to minimise walls, etc for better reception.

As a side note it is not marketing hype.

108 Mbps technology is allowed in the 802.11g standard.

Dlink calls it Extreme G, Linksys calls it Speedbooster.

If you buy the Linksys network adapter with Speedbooster (more expensive than non-speedbooster), then it will connect with your Linksys router at 108 when enabled in the router configuration settings.

Otherwise your stuck with 54 Mbps as Curlyben mentioned.