How fast does my system have to be to use this feature? I got an Athlon 1.20Ghz processor with 512 mb pc100 ram.
Whether or not it will work is dependent on a lot of things such as the IDE data transfer mode, the speed of the source hard drive(s), CPU speed, whether or not the source and destination drives are on the same IDE channel, and more.
Ultimately you're just going to have to try it. You can probably do a simulation with a pair of DVD-R drives to see if it works without wasting media.
Also, if I can burn a disc at 8x in about 8 minutes, does that mean I can burn two discs at the same time for around 8 minutes also?
That is correct.