Actually with a Celeron 2.4 and 256MB, that is
the reason it's so bad. Older Celerons are absolutely horrible at gaming. And that ram isn't helping at all. 256MB is the minimum for XP but it does much better on 512MB. IMHO, you should think about swapping that CPU with a proper P4 and boost that ram if you want to improve your gaming experience.
Notice that even with a 6800GT the Celery 2.4 cannot push more than ~50 FPS in those 2 games...