On Linksys equipment (and maybe others), when WPA is enabled, the power output to the antennas is lessened, therefore making the signal strength lower. I have a WRT54G and had WPA enabled, and it worked fine, but got a very low signal in my bedroom (note: I live in an apartment, so only one wall between my router and room). I disabled the WPA and enabled WEP only, and now the signal strength has significantly improved.
Also, I talked to the Linksys reps at Quakecon this past August in Dallas, TX. They told me that Linksys was going to start putting a little higher grade antennas in their wireless equipment.
Also, you might care to look at
www.linksysinfo.org and
www.sveasoft.com for firmware on your wireless router. Only do that at your own risk though.