That computer's pretty darn new, isn't it?
Your CPU is adequate. If you really want something better you could get a 6000+ @3GHZ or even a 6400+ @3.2GHZ, but I don't think that it's really necessary ATM. They're both around $100. Alternatively you could overclock the one you have by ~400-500MHZ for free. Any more than that would require an aftermarket heatsink for about $25.
You have plenty of memory. Are you using Vista 64bit?
Your video card is crap. Pure crap. This is the only real weak point in your entire system. So let's find you a new video card and power supply to feed it and call it a day. Depending on your monitor's resolution you should be able to get both (and very good ones at that) for ~$200-$300.
What's your monitor's resolution?
Do you prefer ATI or Nvidia?