I agree with all the folks that say there's always going to be something better.
Yeah, Athlon has the socket M comming out, but the Socket 939 still has a future ahead of it. If you want to upgrade to that, that's perfectly fine. If it were Socket 754, I would say no-way, but Socket 939 is fine.
The GeForce 4 card is OK. Not great but you can get better cards now days. The GeForce 4 is going to bog down in DirectX 9 games.
So it comes down to... do you play alot of DirectX 9 games? Do you anticipate buying any upcomming games? If not, don't upgrade. 2.4 GHz is still fine for everyday computing, provided you keep spyware off your PC.
Theoretical upgrade:
Athlon x64 3200+
Socket 939 motherboard with NForce 4 chipset (your choice of SLI or not, personally I would recommend against it for budget conscious).
1 GB DDR 400 RAM
GeForce 7800 GT PCI-Express if you want pure performance (Gigabyte GeForce 6600 GT with the Silent Pipe if you want to save some money and have a QUIET videocard that still performs well).
My rationale for the upgrade is its always better to spend more money on your graphics card than the processor if you do gaming.