WoW's demands have been getting higher since BC was released (especially in Shattrath and other densely-populated zones), so it's possible that what was good enough for the initial release is reaching its limits now.
Upgrading RAM generally makes a huge amount of difference. You didn't say what OS you're running, but if it's a Windows-based laptop, you can right-click on the "My Computer" icon and select "Properties", and that'll tell you how much RAM you've got. I've had serious performance issues with 1GB or under, but 2GB and over seems to make all the difference in the world. The good news is that RAM is generally quite cheap and easy to replace/install -- it's by far the easiest machine upgrade, even on a laptop.