i like that computer very good as a beginning.
are you willing to get a new motherboard if yes then AMD Opterion is the way to go.
get a load of these nvidia 128 Mbs
GeForce4 Ti 4800 Vertices per Second:
Fill Rate...
ok id get a ati radeon the new one.
then id go for a AMD 64bit processor.
and id check into those 2 videocards doing half the work. i mean it can make a big fps change.