Yes, you have to have Linksys Devices (i.e. PC Cards, PCI Cards, etc) that support the SpeedBoost technology to get the full speed.
And it's manufacturer (sp ck) specific. If you go with D-Link, you'll have to upgrade all of PC/PCI cards to the newer D-Link special versions to match the router.
If it's Linksys WRT-54GS, you'll have to upgrade your PC/PCI cards to the Linksys SpeedBoost versions.
Btw, I found the DD-WRT firmware the best of the ones I tested on my WRT54G v3.0.
www.dd-wrt.com