Yes, you have to match wattages. The only notable exception is that 150 and 175 ballasts and lamps are sometimes interchangeable, becuase the wattages are so close, but this is only advisable if the ballast is rated to handle both wattages, which not all are.
I'd also recommend the Radium lamps. I had XM 14K and switch to to Radium 20K at my last change cycle and I coudn't be happier.
120 is right, Radium, Phoenix, Ushio and I think Sunburst require electronic or pulse start magnetic ballasts to run them, so you need to be sure your ballast is as such.
If you do buy the Radiums, buy from a reputable dealer with a well documented return policy. Radiums have a higher than normal DOA rate, a fact that I can attest to. One of my 3 lamps was bad out of the box. That Depot for Marine stuff swapped it out no questions asked, so no biggie.