Ask yourself this question- "Do I want to play D&D in modern day?" If the answer is yes, then go with D20 Modern. If not, I'd strongly encourage some other system, because in the 10 or so times I've played D20 Modern (and Spycraft for that matter), it truly plays like modern-day D&D, even with no magic or supernatural stuff involved. While that is a strength to some folks, to me it was a huge weakness, and turned me off D20 Modern completely.
Savage Worlds would be my recommendation- its incredibly easy and quick to prepare and play. While the cards and bennies seem a little daunting at first to some folks, they become second nature REALLY quickly, and add a lot to the game. In fact, I've spent the last three Saturday nights running demos of SW at my FLGS to people who have never tried SW (and three ladies who never gamed, period). All of them picked up the game within 20 minutes, and had no trouble keeping up. I've used SW for dark fantasy, hard sci-fi, victorian horror, Deadlands, Soloman Kane, Pirates of the Spanish Main, Rippers, Fading Suns, RunePunk, Fallout (post-apocalyptic), zombie survivial horror, and a Silent Hill knockoff, and it worked perfectly every time. Its truly the most versatile and robust system I've ever played.
Second recommendation would be for NWoD, Its a really nice, streamlined system that is considerably darker in tone than SW. While I've only played a total of maybe twelve sessions using it, I really do like the NWoD and would love to use it more in the future. Especially highly recommended is the mortals version of the game, and with sourcebooks like Second Sight (psychics), Antagonists, Haunted Places, and Urban Legends, it would make an awesome modern game of subtle horror.