The last "rare roast" I did was slow roasted at 250� for a few hours. Use a meat thermometer to gauge the temp in the center, but the goal should be about 115�-120�. Pull the roast out, crank the oven to 500� (and put a drip pan in...if you're not using a roasting pan), and put the roast back in for about 10 more minutes. With such a large piece of meat, the internal temp won't be greatly affected, but it'll give a nice brown exterior. The reason I like doing it this way is that most of the center remains rare (pink & juicy), but you still get a thin layer of brown "crust". If you sear it first, there's a thicker layer of grey/brown meat which isn't as desirable in a prime rib.
The above method works very well, but be sure you let the roast dry out, uncovered in the fridge for at least a day. Then salt the crap out of it at least 8 hours before you cook.