That card is pretty low in terms of running RS. Then again it could be your PC is just slow. Make sure you have the latest drivers for your card.
I've been told it runs alot smoother now on the runescape client. Available under wallpapers & download on their website. If this doesn't work, check if you have the newest video drivers and run ccleaner. Alternatively as a last resort, you can reinstall windows with a fresh installation.