In regards to steak rarity: 125F is Rare, 135 is Medium Rare, 145 Medium, 155 Medium Well, and 160+ is Well. No one wants to eat a piece of meat with holesin it, so most chefs do not use a meat thermometer. It is possible to get a fairly accurate temperature by feeling the firmness of the meat.
Hold your hand open and relaxed, palm upwards. Lightly touch the tip ofyour index finger and the tip of your thumb together. Now, feel the mound at the base of your thumb. That is what a rare steak should feel like. To get differing degrees of doneness, simply progress one finger over for each degree. For well done, apply some pressure with your pinky and thumb, to tighten the mound.
I also agree with abadname regarding "sealing in juices." The purpose of searing a piece of meat is to caramelize the sugars present in it, making it more visually appealing, offering a contrasting texture to the preferably rare center, and enhancing the flavor.
As far as a good compound butter, you could try Buerre Poivre. Over high heat, reduce 8oz full-bodied red wine, with 1 clove garlic, 2 shallots, and 3-4oz cracked black peppercorns. When about 1oz of wine is left, remove from heat, strain thewine reduction out, and fold it into 8oz softened butter. If you happen to have any veal stock, I would advocate adding 8oz stock, reducing by half, and then adding 2oz of raw butter off of heat in lieu of folding the wine into the butter, but Veal bones are surprisingly hardto come by, at least here in Atlanta.
