|You can pretty much forget about a "better" motherboard. So far as a graphic card upgrade, it's difficult to make a recommendation without knowing what power supply Sony stuck in that thing. You should find a label on the power supply stating the total wattage and the amperage on the +3.3v, +5v and +12v rails.|
The best AGP video cards will cost you about $100US and those will likely not work with the original power supply. If you just want to replace the MX 460, just look on ebay (for example) for about any 64Mb AGP video card and don't pay more than about 10 bucks.
I'd forget about a motherboard upgrade too; good Socket A boards are still pretty expensive unless you come across a really good deal. You'd probably want to upgrade the processor too; that and a new power supply will add even more to the cost.
Here's the deal; you spend a couple hundred bucks to upgrade a $50 computer and you end up with a faster $50 computer when you're done. And some extra parts worth about nothing.