According to Steve Beniout (sorry steve, if I spelled the last name wrong), the order of quality goes like this: 266 boxed, 266 OEM, 200 Boxed, 200 OEM... and in all reality, the best way to go is with boxed, mainly because you get the 3 year warrenty instead of a 30-90 day one, and apparently, although you already have your hands on a silverado, the HSF that comes with the boxed 1.2 and 1.33 is good enough to take the as high as 1.5, or maybe even more, if you can keep everything stable, as apparently heat won't be an issue. Also, by getting a boxed cpu, you guarentee yourself a copper core, which is the best core you can get, as it conducts heat better, and can scale better, and support overclocking better. so, pay the extra $30 for that piece of mind and the added ability to overclock, if that is what you want... i can't think of any other reason to get a sliverado, unless you really need to quiet fan operation... but thats my two cents...
-SammyBoy
ps. you can also message stable, and he will tell you anything you'd ever want to know about OEM vs. Retail versions