Agree with not doubling the enemy. I would use one of the online encounter calculators. Since I tend to focus on 1 big encounter per session, I usually use the calculator to see where the border between "Very Difficult" and "Overpowering" is... meaning if I added one more orc to this encounter, it will go from the first category to the second.
But, simply doubling the enemy will often not work. A party of EIGHT PCs will often be a lot more diverse in their attack and defense options in combat. Instead of one arcane and one divine caster in a party of four, you might have 2 or 3 of each (wizard, bard, duskblade for arcane; cleric/druid, plus paladin for divine)... instead of 1 frontline fighter, you might have 3 or 4 (fighter, paladin, ranger, barbarian, plus the duskblade)
But, simply doubling the enemy will often not work. A party of EIGHT PCs will often be a lot more diverse in their attack and defense options in combat. Instead of one arcane and one divine caster in a party of four, you might have 2 or 3 of each (wizard, bard, duskblade for arcane; cleric/druid, plus paladin for divine)... instead of 1 frontline fighter, you might have 3 or 4 (fighter, paladin, ranger, barbarian, plus the duskblade)
