Some general advise from what I picked up when I got a new computer roughly a month ago:
What kind of power supply you need depends on what hardware you have in the computer. Use the Power Supply Calculator to get a rough idea of how many watts you need, but also keep an eye on the ampere on the 12V rails since modern graphics cards draw power from there. At least 30A (in total if you have a power supply with two 12V rails) seems to be recommended if you have a decent graphics card (though I have a 450W power supply with 14A on 12V1 and 15A on 12V2 and haven't had any problems with my Radeon X1900 so far). More might be needed if you want to run multiple cards with SLI (nVidia) or Crossfire (ATI).
Since PCs tend to be loaded with cooling fans nowadays it may also be wise to pay extra attention to what type of chassis you get to keep the noise down. I learned this the hard way, and have had to bury my new computer under a mountain of pillows to get the noise down to a level where you can stay in the same room as the computer for more than 5 minutes.
Also, if you intend to get a new monitor as well I'd advise you to stay away from Viewsonic's 19" TFT monitors. I bought one when I got a new computer, but the image/color quality was so poor I had to revert back to use my old CRT monitor with the new computer and relegate the new monitor to be used with my old computer. (That, and Viewsonic's customer support sounds like they want you to hang up already so they can go back to sleep...)