Agood advice, but don´t deviate too much... many encounters consist of 4 or more enemies. You can easily mix elite, standard and minion monsters.
A good rule of thumb is making classed and named monsters elite (applying a class template per DMG rules)
In this way, 3 orcs may pose a challenge.
I thought the same when I began converting adventures, and quickly found this to be a poor solution for a number of reasons. I'll elaborate.
First off, applying class templates is a fair amount of work. Not only do you have to go through the normal work of applying a template, you must also select class powers from the class you're adding to the monster. Class templates already add a huge number of powers, so class-templated monsters end up with huge stat blocks - far larger than they need to be.
Second, class templates are not as effective as normal monster templates. They add versatility, but do not provide the monster with the level of round-to-round power that it ought to have as an elite.
Third, remember that players aren't necessarily going to have any idea what powers the monster has. Sure, if a PC is of the same class as the monster, he or she might notice some similarities, but typically they'll have no idea.
Given this, adding class templates rarely improves the players' experience, rarely improves the DM's experience, and creates more work for the DM than is necessary. The urge to apply class templates can stem from the desire to create as faithful a conversion as possible, but conversions are always better off when they preserve the
spirit of the original adventure rather than the letter.
Class templates have their place, but they ought to be used sparingly. Templates, in general, ought to be used slightly more. The majority of the time, when you want to include an elite monster, you should just find a working, pre-made elite monster in the Compendium, or find a standard monster you like and an elite monster with some cool powers, and snag some of the elite monster's powers for your standard monster to use (while upgrading him to elite).