I believe on linksys router you have an option to genetate 4 keys or only one key. If you generate 4 keys, the access point will be able to authenticate ( if using shared key) and encrypt with any of the 4 keys... So it up to you which one you wanna enter in the client config.. But Access points generally transmit only the first key...
4 keys are more useful in bigger n/w say if you have 100 clients, with 4 keys you can segment the network in to four, that way if one of the key is compromised you will not have to change keys in all the clients , just the ones that were on the segment.....just some extra security.....

As far as I think, you don;t need to enable 4 keys in a home n/w were theres only 3-4 comps and its not that hard to change the keys on clients if compromised.....

About you having bad signal, that you need to find out what all obstructions you got in the signal path.... say for eg: cordless phones(wireless-G and cordless phones use same ISM band 2.4ghz), or metal surfaces etc....

hope this helps......