Hello and Welcome to the forums!
The 2008 Harpertown single 2.8 does support installation of another processor which would then make your 2.8 an 8-core machine. You would need to get the same heatsink and processor + get arctic silver heatsink compound. The processor, as you found and discovered on ebay can be had for 300 or less.. the heatsink I would advise you to visit:
www.applecomponents.com - The person who owns this company is really nice and will assist you in anyway. I always use them for lesser, known parts for my G5 and or Mac Pro. Some of his prices are a little off the wall while most are reasonable. Check them out for the heatsink. He might also have the 2.8 E5462 Processor as well.
Good luck.. I had the Quad-core 3.0 version of the 2008 mac pro and its a solid performer.
I have the same setup and am going to do the same upgrade just needed some more info.
Do I need to do anything to the system other than installing the second proccesor? Like maybe reinstalling OS or something to let the system know it has a second proccesor? I'm guessing it's like installing RAM? Also, I was checking on eBay and found several E5462 proccesors. Does any E5462 work? Some say HP, LGA 771 or SLANT Costa Rica. Are these all the same? Oh, Could I pull out the 2.8GHZ chip and through in two 3.0GHZ chips. If yes which 3.0 Prossecor I am looking for?
Thanks for the original post and for reading and asnwering this one.