What are you trying to fix?
If it works great as you said are you trying to break it?
Your vid card is pretty far down on the scale but if the old one works ok for your use why upgrade.
Be aware that if you do deceide to start upgrading you might run into issues like power and others, the can of worms gets opened.
384 meg ddr ram ,40g hard drive, amd athalon 1.4 gig processor , running windows me , nvidia riva tnt2pro graphics , sb pci(wdm) sound , cd writer, dvd writer, and dvd rom drives. custom build 2 years ago, still runs great. use it mainly for gaming, web/ downloading, burning cds/movies, playing movies, and encoding. so whats the best thing to upgrade first?