Macbook Pro Running Slow

I recently bought a Macbook Pro 13in. it's the one that cost $1,200. This is the first Mac I've ever owned and I've had it for a few months. At first I absolutely loved it, all the cool programs I had never used and just how smoothly everything operated. I've downloaded quite a few additional programs and also have put thousands of photos, a few home videos (baseball games), TV shows, music etc. on my unit. Recently while making a end of the season slideshow/video on imovie then using iDVD to burn the video my mac has has an extreme drop in performance. imovie is useless to me as it takes forever to load and if I try and add a new clip audio or video it takes upwards of a minute. Also some time Safari will unexpectedly quit responding. I'm so sick of seeing that little color wheel load thing haha. So id there anything I can do to increase performance? Or have I just maxed out the core duo? If so does anyone now if theres a way to trade up to the 15in? Thanks, sorry for my ignorance.

-Mike