Update: Just ran N-Bench and my great almighty expensive graphics card (well it was at the time) is only just beating a Geforce 2! In fact in the Star Fighter demo it got beaten by it! My results were:
Ninja 1: 1209
Ninja 2: 1366
Grand Touring: 1244
Star Fighter: 816 (there was a lot of screen corruption in this!)
3D Overall: 1158
Any ideas what could be wrong?
Edit: screenshot of artifacts here http://notacake.com/ds2.jpg