you will be able to understand the plot fine and you don't have to play Uncharted 1 to understand the plot or anything in Uncharted 2. But even with that in mind you should do yourself a huge favor and still play the first one first. It's just an incrediable game.
Just like the other two respondents said, you do not need to play Uncharted to understand the plot of Uncharted 2. Uncharted 2 really doesn't reference the first game at all, except for having a few of the same characters. That said, both games are amazing, and it's well worth your time and money to buy and play them both. I bought them both after Uncharted 2 came out, and played them back to back, and had a grand time, and can't wait to go back and play them both again.
There's not reason not to own both, in my opinion. The price is down quite a bit, you could get them both brand new for the price of one newly released game. Also, if you play Uncharted 2 first, then going back to the original will be a little difficult. Uncharted 2 is clearly supreme to the first.. Uncharted 1 is still a great game however, so it's worth playing that one first, then going to Uncharted 2. You'll appreciate Uncharted 2 much more, also. You'll see just how much more was added, and how incredible it is.
You should play the first one before the second one. Generally, you may understand most of the key elements of the plot in Among Thieves, but you may have a hard time understanding the less important details that were established in Drake's Fortune.
PS: In terms of how good each game is, BUY BOTH!!!!!!! YOU NEED TO BUY BOTH!!!!!!!! I DON'T CARE IF YOU HAVE DOUBTS, GET BOTH GAMES!!!!!!!!!!!!!!!!!!!!!!!