I was wondering ~ These pre-release press kits (White labels)... Are they actually worth anything? Obviously there are collector's out there who collect white labels, but what about N-Gage NFR ones?

I was recently given one of these press kits for Sytem Rush, and was wondering how limited they actually are? Obviously they are fairly hard to come by unless you are lucky enough to work for the press (Or look on eBay), but are some rarer than others?

Also, what is the difference between a press kit and a "Gold Master Review Kit"?