Yes The X1900XTX requires more due to the cooling and size of the card compared to the Nvidia's. As far as any card goes with Wattage the minimum requirements for Power Usage can be found on the Packaging or Product Details. I would go a 100w over minimum requirements, however. To prevent your stock 300W PSU from smoking....oh that was me.
If your running minmum watts dont expect your PSU to last forever. And be wary of inexpensive brands, go Thermalake if you got the $$$. As far as cheap products. My freind uses Echostar and it never let him down with any of his rigs. The Powork I have is a good price for the wattage and has lasted my a few months so far.
It seems to me
256mb memory = 350w min require
512mb memory = 450w min require