Forgot to mention that I do have a 600w PSU.
I'm more concerned with what the package could use, then deciding on a total investment.
The Phenom 9550 is only 2.2Ghz which is the same speed as the CPU it has now. I am looking to replace it with something faster. At least something with 2.8Ghz.
After finding the new CPU I want to find out how good of a graphics card I will be able to add to the package. Something like a Radeon 6670 or even a 6770. But not as extreme as the 6870.
the link tells you what processors you can upgrade to. if you can flash the BIOS perhaps you can give it an upgrade.( real upgrade ) you'll have to check with the board manufacturers site for that ( ECS ). having said that, it's extremely limited and putting another processor on it ( as things are ) won't show much difference. you should seriously think about a major overhaul.
the video card would make a big difference for you if you were using the on board video. you should think of upgrading that first and see how things go.
it also says 3gig of memory. running Vista you should get 2 2gig sticks so it'll run better.
you have an hp pavillion its likey you wont be able to upgrade it unless you buy hp parts at a premium. if you dont want to do this you may have to replace the motherboard and memory. as they often are made hardware dependent for hp specifically via bios and what i call a caliper chip.(restricts what can be replaced, if it detects to much change in hardware it forces a shut down. there is nothing wrong with the pc. just hp dont like you adding to there oem builds because they want to charge you for them to do it.
also hp are known for under powering there oem builds. so its likely your psu will handle what you currently have but wont hndle say a bigger gfx card or cpu. meaning your pretty much gonna have to guy that pc to make a decent rig from its bare bones.