Decent enough rig, but there is always room for improvement now matter what computer you have. The thing is are you happy with performance and if not how much are you willing to spend to improve it? Also, unless you are gaming, there is no need to upgrade your graphics, but if you are then that needs to be addressed.
For your CPU, it's decent and can be overclocked easily with your current motherboard. It all depends on how good a cooling setup you have. If you are using the stock heatsink, you aren't going to be overclocking very far. If you go with a good air cooling solution, you can get to 3.6 to 3.8 Ghz fairly easily and maybe higher. With water you could do 4.0 Ghz or higher possibly. Overclocking is more art than science. It's not complicated at all, but it will take a few hours of slowly working with your system to get a nice stable max overclock with your system. Basically look for what others have done with your current motherboard and similar chip based on your cooling solution. If you see others with an air solution for example get your chip + motherboard up to say 3.8Ghz often enough, start by increasing it to 3.5Ghz first. Test it with prime95 Small FFTs for an hour. If that works, bump it up to 3.6, then 3.65, then 3.7, then 3.725 and go so in smaller and smaller increments. Run a temp monitor program to make sure you are staying within a decent range of heat for your chip. Usually 80C or less when running prime95. Also increase the voltages as needed while overclocking to keep things stable.
You can also go with more memory as well as 2GB is okay, but not all that great.
Another huge performance boost would be to upgrade the hard drives. The prevailing opinion of most people, myself included, is to grab a nice solid state disk (SSD) drive and a good size fast mechanical drive. 60GB or more for the SSD is more than adequate depending upon how much you are willing to spend. Install your operating system and programs you want to run on the SSD and put everything else on the back up mechanical drive. Try to also get a nice mechanical drive with 500 GB density platters.
The power supply you have is OKAY, but could be better. If you don't plan on doing a multiple video card setup in SLI or Crossfire then you do not need more than a 500 to 600 watt power supply. A good quality power supply shouldn't run you much in that wattage range. If you do want to try a multiple video card setup, you may need more power depending upon the cards you are using. For example, if you have the money to splurge on three GTX 480's you might want to consider a 1100 watter or even higher power supply.
If you are into gaming and need a nice performance boost for that, a good graphics card is called for. The fastest single GPU solution is the GTX 480 nvidia card, but man it sure is expensive. A nice alternative is a 5870 card. If that price is still too expensive, there are many good lower bracket cards out there. These are the cards I would look into based on prices
sub $100 = ATI 4850 (with reference cooler and heatsinks on the memory) make sure you get a card with heatsinks on the memory though
sub $150 = ATI 5770 (power cooler brand from zipzoomfly is $135 right now)
sub $200 = dual ATI 4850's or ATI 4890 for single card
sub $300 = dual ATI 5770's or ATI 5850 for single card
sub $350 = GTX 470 nvidia
sub $400 = dual ATI 4890s or ATI 5870 for single card
over $400 = if you have this much to spend you probably already know what you want.
Last but not least, upgrade to windows 7. If you plan on having 4 GB or more of memory get the 64 bit version.
That's about it for general performance increases.