A few thoughts--
CPU: For what those chips go for, you probably don't want to upgrade. New chips for the same price are far better. Mind you, what you have isn't too shabby. Why not overclock it a bit? That's free (unless you screw up really bad and fry your chip). You may find that $150 or so for a marginally faster dual core isn't really worth it.
PSU: You very well may need a better PSU if you plan to upgrade the video card substantially. Make sure to check the requirements for the card and take into consideration that older PSUs can't put out like they did when they were young.
RAM: If you upgrade to 4 GB, you OS might not recognize all of it. 32 bit OS's can only see 4 GB total including virtual memory. You can turn virtual memory off, but some programs will refuse to work without it. I don't know off the top of my head is your CPU can even run a 64 bit OS.
GPU: Again, make sure to check the power requirements. Most decent new cards require extra power cables also. One potential deal breaker is the PCIE revision. I couldn't find it in the spec and judging by its age it could only be 1.0 or 1.1. If it's 1.0, it will take SOME 2.0 cards. If it's 1.1, it will take any 2.0 card. If it's 2.0, it will take any card. There was a change in the power requirements between 2.0 and 2.1 cards that broke compatibility, and so the older slots can't take the newer cards. If you call ASUS, they'll be able to confirm what revision it is. You can try asking a friend to let you borrow one to test it.