I talk with a lot of people about getting started with standing desks, and I usually recommend throwing a small cheap table (ikea) on top of your existing desk and getting a drafting chair (tall). This way you don't have to pay for a variable height desk, and you keep it on the cheap side to see if you actually like it. Also, be sure to start slow and stand for maybe an hour or two a day for a few weeks. Then gradually stand more and more.

I've gone into more detail here: http://reustle.io/blog/cant-stand-sitting