AE CS3 rendered movies are slow and choppy...

Hello, we're new to AE, so I apologize if this topic's already been covered. I convinced the boss-types to invest in Adobe's CS3 package... we are very impressed with AE CS3, except for one big problem: when I render a movie, it plays slow and choppy... and I think it's the way it's rendered, because dropping it into Premiere Pro, it play choppy there, too... I'e tired increasing the amount of RAM used (3GB available on the machine), but I'm not sure I've done this correctly... the machine is a PC, dual-core 2.33 Ghz, with an NVIDIA 256MB video card... why won't AE render the movies smoother? (they've all been 30-seconds or under, by the way)

Thanks in advance for your help!