Video card upgrade will make a huge difference in 3D games. An ATI Radeon 9700Pro would be my pick. But if you dont play 3D games then its not so much an issue. What do you use your computer for?
Here's a benchmark link that has game benchmarks on pages 5 thru 20.
http://www.anandtech.com/video/showdoc.html?i=1904&p=1
Depending on your OS and how stable your motherboard is with adding another bank of memory, you may want to make it a total of 1024MB. Nothing wrong with 512 tho.
If you have onboard or SBLive sound, then I would recommend TurtleBeach Santa Cruz sound card.