I filled an I-130 for my wife back in 2006 as a permanent resident, and recently (march 2008) I've become a US citizen, so I called USCIS and upgraded my petition to the category of US citizen filling for spouse, which the processing dates (July 2007) are far passed my priority date( Feb. 2006). but every time I call USCIS, the representative will give information that are completely different from what the previous one had said.
so which date do I go by in terms of monitoring the processing dates online, the priority date on my receipt ( Feb. 2006) or the date I upgraded my application to a US citizen ( march 2008)?

Since you filed as an LPR, so the receipt date of Feb 2006 is your priority date as an LPR. After you upgrade your status as a USC, so the date of March 2008 is your priority date now. You can't beat the people that has already applied a USC before you (July 2007), right?
So, you are almost there. several months to go, congratulation!