Blind troubleshooting wifi problems is more a magician's task rather than a technician's if you ask me

When I had my WRT54G running with ESSID disabled & WPA I had lots of disconnect problems with the Linksys PCCard. As soon as I returned to WEP & turned ESSID broadcast on the problems dissappeared.
Since then I have added a Zyxel G100 card and a Dell D600 laptop with built-in broadcom adapter into the mix. The G100 card works best with XP's wireless support disabled, while the DELL won't do a thing unless it is through XP's wireless support mgr. (Symptom : radio utility has "excellent" reception, but you do not get a DHCP address and no traffic flow is possible).
G110 cards are not (yet) available in Belgium, so a Zywall/Prestige unit that is made wireless by adding a G100 is only usefull if the wireless clients are in the same room.
If you intend to use roaming, make sure you verify with the vendor that your AP/client config actually support this (and stick with a single vendor for your AP's).
Last but not least : a lot of people seem as happy with Buffalo Tech for wireless stuff as most forum members here are with Zyxel for router/firewall/VPN appliances.
(In fact the Belgium distributor for Zyxel recommends Buffalo rather than Zyxel for wireless products, but his motives may be more commercial than technical).