Straight MDs can do research, but their form of "research" is typically historical/stats type stuff. Like for instance asking the question "Does treatment X correlate with outcome Y?" and then digging up the statistics from published medical data to find the answer. For regular MDs to go into "real" (i.e. experimental) research, you'll have to typically do some type of research fellowship to get into the game.
You're definitely not the only one wrangling with this issue tho...there is some good debate about whether (for someone who knows they want to go into lab research) it is better to go MD/PhD or MD+research fellowship.
I'd search around the
physician scientist forum, cause I remember there were some good threads on this topic.