The specs for your pavillion can be found here:
http://bizsupport1.austin.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c01892905&lang=en&cc=us&contentType=SupportFAQ&prodSeriesId=3999459&prodTypeId=12454
I'm quite certain that pavillion has the same case as mine and so you can fit most likely any standard ATX PSU in it. Also consider GPU length. Check the dimensions on that spec site or take a ruler. Some of the really long GPU's wont fit. I believe any graphics card longer than my EVGA GTX 560 Ti won't fit.
Your system is very similar to mine. I wouldn't bother upgrading the CPU. I don't think you can really put in a CPU under which you would notice that much of a difference. Perhaps you can try overclocking your existing CPU. You have the same CPU as me - I haven't tried overclocking it yet but have been thinking about it. My system has a little more room CPU wise (I probbaly could invest some money and see a decent difference but I still dont think its worth it)
It would be nice if you buy a GPU/PSU you could eventually carry forward into a new system.
Check out a thread I created a while back before I did more research and settled on a purchase:
http://www.tomshardware.com/forum/345865-33-athlon
If I could go back and had more money I would have grabbed a Corsair HX850W PSU so that I could eventually carry it forward into a new SLI system. I also might upgrade my video card in the next 90 days through the evga step up program: http://www.evga.com/support/stepup/
Then in a year or 2 I would buy a new mobo/cpu/memory and drop my psu/gpu into a new system. I still might do this but if I had the 850W PSU I could further upgrade the new system with an additional GPU in SLI.