if you have that setup, you can use one hot shoe cable to extend the reach of the 580 and place the 430 wherever you want within 30 feet or so like your allready doing. Just do that for now unless you want to spend 200$ + for proper transmitters and recievers that are not IR but RF will go further and around corners and through things. pocket wizzards, elinchrom skyports, these are both the only ones I receommend from personal experience.
get the cable I think the expensive one is 40-50 bucks, maybe there is a good 3rd party one out there a little less, even if you get the transcievers you can always use the cable to mount the main flash high up on a bracket.
try a tripod when your shooting dusk, itll help you plan more ,
and make sure your shutter speed is below 250 or youll get black bars the synch speed on the 20D is 250 for most flash units
Bring a ND#4 filter or higher with you, if your having problems getting your shutter speed below 250. But try using narrower Fstop first, like F5.6 - F9 if still not able to get shutter down to 250 then make sure your at iso 100, then if still not slow enough put on the ND, you might also need that if you want to use external flash and also small Depth of Field at the same time.
Whatever you do, in my opinion dont get the optical triggers that go for cheap on
ebay. If you need to rely on this really they will malfunction sometime when you need them, and other camera flashes can set them off, and the distance is not too far. Save your money and pick up real transmitters wether you decide to go canon IR, or PW RF type setup get a decent system itll last you 30 times longer and you can also use it to shoot events, parties, and weddings.
You cant rely on optical slaves when you have other cameras around shooting off flashes too..