1. you will have to re-install if you want a stable system. sometimes XP(SP2) will work on a different system without doing anything to it, sometimes you perform a "repair" instal to overwrite system files, but the best way it to format and clean install.
2. in single channel memory mode, a second RAM stick is not used until the first stick is maxed out (which is not very efficient). in dual channel memory mode both sticks are utilized at the same time (basically commands waiting to be executed do not have to wait for the previous command before it can start, think of it as: single channel= long line of shoppers and only one register open... dual channel= same shoppers but two registers open... the latter is more efficient)
3. Its not a question of the mobo supports it or not, the mobo doesn't really matter because AMD K8 processors have the memory controller built into the CPU. socket754 only supports single channel, socket939 supports dual channel (which is why it needs the extra pins)
4. AGP was the way to go years ago (AGP has been around since the PentiumII days). PCI-E is the current standard. you can still get some very nice AGP cards, but the interface is being phased out. most socket939 boards and probably all of the AM2 boards will have PCI-E as the video standard.