DD-WRT HELP how do i do this

DDWRT have their own forum for this which will help you better. Also its as simple as flashing the router then set it to AP repeater. Just note that the WRT54G will only repeat the signals it supports so if it does not support N and AG then it will not repeat it.

The only way for the wrt54g to have a different SSID (in repeater mode) from the Fios wireless would be if it had support for dual band wireless, which it does not. The only way to get a separate SSID in your case would be to use your wrt54g as a wireless bridge and then hook your Netgear router to your Linksys router with an ethernet cable and have the Netgear broadcast a different SSID and password.

They can use different SSID's if you wire them together. This way the communication between the routers is on the wired network and they are free to use whatever SSID they want on their wireless networks. But if you want the two routers to talk together using wireless they have to use the same SSID, password, frequency and channel.
Think of wireless Ethernet like a CB radio (or walkie-talkie). Only one person can talk at a time and you can only talk or listen at one time but not both. That is exactly how wireless network radios work. So if you have 5 devices in your house using your wireless that is like having 5 people using CB radio's together. Only one person can talk at a time. Lets say your neighbors also use CB radios. Lets say they use the same channel as your CB radio's. Lets say they also have 5 people talking. Now you can only talk when the other 9 people are not talking. This is how you get interference from your neighbors on WIFI networks if they are on the same channel. Now your Wifi is encrypted so your neighbors can't get your data. So for the CB analogy, lets say your neighbors use a language you cannot understand and you use one they can't understand. So your conversations are still private but you have to share CB airtime with them because they are on the same channel.
Now back to the original question. Why you can't use a different SSID (and to add, why does repeating a wireless signal cut your bandwidth in half). Back to the CB radio reference. To talk to someone on a CB you need to be on the same channel as them and talk their language (which on a wireless network to speak the "language" you need the SSID and password). So lets say I am in the middle with my radio. You are a mile to my west and the person you want to speak with is a mile to my east (so 2 miles from you). Your radio won't reach him but my radio can reach both of you. So you want to relay (repeat) your message through me to the guy at the far east. To hear you I have to be on your channel and talking your language. You tell me what to say and then I repeat it to the guy to my east. We are all on the same channel and talk the same language. Now since I have to repeat everything you say, it takes twice as long for you to talk to the guy to my east as it would if you could speak to him directly (because I have to relay it). This "delay" is what causes Wifi networks to cut the bandwidth in half in repeater mode.
But if you called me on the telephone (Ethernet cable between us) and told me what you wanted me to say, then my CB radio and your CB radio do not have to be on the same channel (and SSID). And even more if I held the CB up to the phone you could talk directly to the other guy and you would not have the delay associated with repeating. Thus when you wire your two routers together you don't get your bandwidth cut in half as you do when repeating.
The CB radio is not a perfect example but it gives a person a very good idea of how wireless Ethernet works.