Upgrade means how far do u have in budget.
If u are planning to get an Athlon XP 3000+(which is quite hard to find) and to change ur mobo to Nforce2 based mobos to overclock it, i suppose u'd be spending around RM300 or maybe more.
So, why don't u just go for maybe those old S754 A64 for almost the same price. That would be better, but u might need to get some ddr rams if u dun have them.
The GC is still decent for light gaming, but it's just really decent, nothing more than that. Dun expect to play all the latest games even after OC as some games nowadays require technology which the card doesn't have.