S.C.
I would recommend a gate at both the top and bottom of the stairs. As soon as they can stand upright, they seem to want to crawl upstairs! I don't know the brand name of our bottom-of-the-stair gate, but we got it at Babies 'R Us, and it's one of those which you can configure to different shapes by adding on sections. The gate we had at the top of the stairs (now on our daughter's bedroom) is a step-on, pressure mounted gate, also bought at Babies 'R Us. We've never had any problems with it, however, I was told by a safety expert that pressure-mounted gates are not the safest because they can fall.
With our son, now age 4, we never put a gate on his bedroom door because ours was right next door and we wanted him to be able to come into our room if he needed us. However, with our 20-month-old (much more active) daughter, we put a gate on her bedroom door to keep our son and cat out when she was a tiny baby, and now we use it to keep her in when we need a second without having to chase her around, or to separate her and her brother when needed.
Hope that helps!