This really could be any number of problems. It also depends what Video/Audio playback your talking about (Flash, MP4, MP3, etc). Also what kind of Graphics card do you have (or do you even have one, most laptops use integrated graphics). If you could provide more information (like computer model, graphics card, etc) we might be able to help more, but for now I'll give you my best guess.
Forget about Flash, it sucks! Flash can be a resource hog, however if you want to browse the web it's kind of important to have. What browser are you using? This can make a huge difference as Firefox can have multiple addons that provide Flash support. You can use Adobe's proprietary implementation, or you can use a non-proprietary implementation. I've always had better success with Adobe's versions (but that's just me).
Google Chrome on the other hand comes with Flash support. (Note: Chromium which is the open-source implementation of Chrome does not come with Flash as far as I know). I currently run Chrome on Ubuntu 10.10 and the Flash works amazingly well (and I have a integrated graphics). So far Chrome or Flash has not crashed once.
MP3/MP4/Video/Audio Playback: To ensure you get the best possible playback I would recommend making sure you have FFmpeg. It supports just about every format you'll use (QuickTime, MP4, Real Player, Windows Media Player related codecs, etc).
Once you have this installed these programs/drivers the only thing that will cause choppy playback is your graphics card, or having other programs running that are using your graphics card (Compiz for example).
If you do have an integrated graphics card, increasing the amount of RAM in your system can help. Increasing RAM is also one of the cheapest upgrades you can do. If you have a Desktop, you most likely have a Graphics Card, in which case you can get a new one that will support better video playback.