Just a quick note - you actually can knit tin the round using straight needles - I'm doing it at the moment. You just need at least 4 needles and you divide your stitches up between all but one of them and knit with the remaining one. It's not as awkward as it sounds (except when you're casting on, which is a bit of a pain). Round needles (which I've never avtually used) are presumably fine when you're knitting a tube (e.g. socks or a seamless top) but I understand they're a bit of a nightmare if you're knitting a flat circle (e.g. a table cloth) because as your knitting grows you have to constantly swap to a slightly larger (longer) set of needles.