I'm going to echo some of the other responses by saying that for $2K you are going to be limited in your options. Blue Nile offers simple Solitaire settings for $250 in white or yellow gold. I would recommend the classic yellow gold as it will allow you to sacrifice the color of the diamond for clarity.
So, you need to do some reading on the 4C's. I recommend checking out this site:
Nice Ice
The site itself is really poorly put together, I'll admit that, but they seem to be extremely knowledgable regarding diamonds. Scroll down and look for the section on the side bar on Diamond Grading (The Expanded 4Cs). Read as much as you can before making any shopping decisions. The more info you have, the better off you will be.
Ultimately you will want to know what your girl prefers: A smaller diamond with no visible flaws and lots of sparkle, or a big rock. It is a balancing act between the 4Cs: Cut, Clarity, Color, and Carat. The bigger rock you want on your budget, the more flaws you will have to accept.
If I were you, I'd get a yellow gold setting. This will allow you to sacrifice color for the other 3Cs. Blue Nile has a 0.68 carat diamond (nice size) with ideal cut, J color, and SI1 clarity for $1439. Ideal cut is very important as it will make the diamond sparkle with lots of "fire", so it will be easy to notice. The SI1 clarity means that there are inclusions and flaws that are easily visable under 10x magnification, but may not be visable to the naked eye. The key word here is *may*. If you were to buy this diamond, I'd call and speak to someone about it to make sure it is "eye clean". Finally, the J color means that it is very faintly yellow. However, if you stick it in a yellow gold setting, no one will know the difference! For the price, you could even select a more fancy setting, something with side stones or channel set diamonds for instance. This would enhance the overall look of the ring and be very impressive.
Blue Nile, and other sites, have systems set up that allow you to pick your diamond from inventory and then "configure" your ring by viewing all the available settings. Play around with this system to give yourself an idea of price and what the ring will look like in the end. Also, do as much reading as possible! The more you know, the less likely you will be to buy a crappy diamond.
Ultimately you'll want to figure out what your girl wants. If she hasn't told you, look at the rest of the jewelry she wears. Is it mainly yellow gold, or does she like to wear white gold (or silver)? Has she dropped any hints as to what shape of diamond she wants? Round is the classic, but there are tons of others. Read up on them, then take a look as to what shaped stones your girl wears.
The things I suppose I want to stress are:
Cut: Get as good of a cut as you can afford. Ideal would be best, but I would stick with at least "very good" (or some equivalent).
Clarity: At your price range, make sure the stone is "eye clean". This means that no flaws or inclusions are visable to the naked eye. This could mean that the stone is SI1 in clarity, or VSI1 in clarity.
Color: As opposed to other posters, I feel that you can sacrifice here to gain a lot of ground in the other catagories. I would go with something that is H, I, or J in color, and then put it in a yellow gold setting. The yellow gold setting will distract from the color of the diamond.
Carat: Get as big of a diamond as your budget will allow without sacrificing more than you are willing to in the other catagories.
Other people will take this the other way around and put size first, but I think diamond size should go last unless your girl has told you that she wants a specific size. I have a friend who told her boyfriend she wanted a 1 carat, princess cut diamond in a platinum setting. It was up to him to balance everything else. Since she wanted platinum, he needed good color. With a 1 carat diamond, he needed good clarity too. It ended up that he spent an arm and a leg for the ring...
Anyway, good luck with whatever you choose.
R