First off, the 8-core MacPro is just a rumor, so I wouldn't put too much effort into thinking about it at this point in time.
As far as the memory goes, I HIGHLY recommend you forget the thought of using non-certified (no heatsink) memory in the MacPro. You WILL regret it. Now you certainly don't have to buy RAM from Apple. I purchased my extra RAM from
MacSolutions. The RAM is Apple-certified, without the Apple price tag. Though I gotta say that the difference when I bought it was nominal. The MacPro requires expensive, non-standard RAM chips - the volume of sales simply prevents this type of RAM from enjoying lower prices.
You will have to use matched pairs (two of the same size chips at a time) or the MacPro probably (though some have reported otherwise) won't recognize one of them.