I'd like to chime in and say that you definitely want to make sure that you've got enough space on your motherboard and inside your case for some of the bigger graphics cards. As someone who had to learn that the hard way, trust me, it's worth it to get something extra spacious.
Honestly though, unless you want to go all out, you're not going to need the biggest and best graphics card. Outside of Witcher 2, Crysis, and a few other key games, a graphics card in the $70-100 range will be enough to let you play most games on med-high settings, depending on your set-up.
And yeah, you definitely don't need 2 graphics cards. Some games don't even support the feature, so I think it's a waste, personally.