Hot-swap HD question
Number of SATA slots should be listed on motherboard description. PC description may list MOBO model. Cheap HP/Comaq etc PCs most likely have crappy PSY, which may not have enough connectors to power as many HDs as you want to add. I build last 4 PCs i had and i remember maintaining/upgrading my parents HP was royal PITA. If going with retail PC, have they configure it for you.