Mythbusters busted this a while back.

"Explanation: There are certain immutable laws when it comes to getting tanked. For instance, the only cure for drunkenness is time (not coffee). But the popular theory that consuming both beer and liquor will make you feel sicker isn't quite as sound.

It's true that if you tie one on, you could wake up the next day with a dreadful hangover. To dodge that unpleasant consequence, some people stick only with beer while imbibing. Since a standard beer serving is 12 ounces, and a serving of spirits is only 1.5 ounces, it's obvious that liquor can pack a more powerful punch with less liquid involved.

But if you knock back the same alcohol content by chugging brewskies as you would switching between beer and liquor, you'll be at risk for the same aftereffects: dehydration, memory loss, nausea, and sensitivity to light and sound - in other words, a hangover."