They should run dual channel, if the timings are different, they'll just default to the slowest stick.
Would it be worth it? Depends... dual channel with Athlon XP's doesn't do much because the FSB is still only 333-400 MHz even though the memory bandwidth is double that with dual channel. It helps a little bit because then the CPU doesn't have to share bandwidth with any devices that use DMA.
If you actually do things that need more than 1 GB of RAM, by all means, get a second stick. Otherwise, you're not going to see much benefit.
And... system RAM has nothing to do with SLI.. so yes, you could still keep the memory.