Need help with first time upgrade/build

Hey guys first post here!

To start, I made a BIG mistake. I bought an HP "Media Center computer" a few years ago. I am living with the pain of the system, and instead of trying to video edit and trying to play games, I am finally going to upgrade. I would like a new processor (would be willing to overclock)/motherboard and maybe RAM if you think it is needed. I am new to this stuff, but have decent knowledge of computers. My use of this machine is probably 40% Gaming, 40% HD Video editing and 20% school/ internet use (the basics).
Here are the specs for the machine I purchased (for a a ridiculous amount...) I already upgraded my GFX card to a 8800GT overclocked, and power supply to a Antec Earthwatts 500 watt (80 plus) power supply. I also would like to know if I should upgrade my case while I'm at it (they arent too much, and I wouldn't mind a cool one!)

My budget would be <$300 but I would be willing to pay more if it is really worth it.

Sorry if this is a lot to ask for, but there are so many options!

Here are the specs:
http://h10025.www1.hp.com/ewfrf/wc [...] R1002_USEN

Thanks for the help!

for around $300 dollars I would look at P45 systems and maybe an e5*00 or e7*00 from itel. Afraid I'm not really in the know about AMD systems. (really should be by now!)