system restore is fine and so is your amount of disc space provided for it,it is not bad for your system and doesnt make any difference to speed,i personally dont allow that much space as i prefer to do regular backups of my own on cdr or dvd,everything i have on the pc i also have on disc,system restore is fine but absolutely no use if your hard disc fails,hence the best backup is cdr or dvd as you can restore them to a new hard disc if needed