..it's built into the motherboard itself. If you wanted it you'd have to buy a motherboard that has it.
I'd be sure to check for compatibility before splashing out any cash for a new mobo though. If your current mobo is only PCI then i'd say the rest of your system is outdated too.
A general rule to follow is that if the price of upgrading your system if 50% or more of the cost of a new computer...you're much better off buying the new machine.
Okay, considering your graphics card i'd guess that an athlon and not a duron.
I'd say you're okay for a while. You also shouldn't have any problems with a new motherboard. And your motherboard has an AGP slot too. The graphics card you have requires it if i'm not mistaken. You're prolly running sdram. If you're dead set on a new mobo i'd recommend making sure it runs RDRAM (you'll need to replace your ram too, but you'll get a big performance boost from that alone)