Hey there, I had previously done some research on a similar subject when outfitting my then gaming PC. When it comes to the difference in voltage, there really isnt any difference in performance. The 65w version obviously uses less power and runs much cooler. The power consumption is somewhat negligible and unless you use the PC for solid gaming or it is in a very poorly ventilated area, the higher operating temp is a non-issue.
However the problem you are going to run into is motherboard compatibility. Seeing as how you PC most likely came equipped with a 4800+, you are going to have a AM2 motherboard. While the AM2 chipset is technically capable of running a quad core phenom processor, the results are a mixed bag. In most cases, if your going to stick with the AM2 chipset, your better off going with a higher end Dual Core processor.
I would personally recommend a Athlon x2 6000+. Its cheap and ist performance gained would most likely be greater than dropping a phenom into your AM2 socket. It's also what I originally used when I built my gaming PC last year. Although I dont really use it for gaming anymore, it is a multimedia monster.
Here on some links to help you out;
(deals with Phenom + AM2 chipset compatibly issues and fixes)
http://www.tomshardware.com/reviews/phenom-motherboards,1749.html(performance benchmarks comparing the less powerfull x2 5600+ to Phenom 9900+)
http://www.tomshardware.com/reviews/amd-phenom-athlon-64-x2,1746-11.html(performance benchmarks, comparing the X2 6000+ to other processors including Intel chips)
http://xtreview.com/review177.htm
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and I was cruising hotjobs.com looking for a new one when I came across this posting;
