So I guess it is useless to do the upres with Photoshop? unless he driver interpolates with lower quality than Photoshop ...
Miguel
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You can probably do better in Photoshop, most print drivers use a simple "nearest neighbor" algorithm for interpolation. But to my mind the more important issue is final print sharpening. Print sharpening should be the very last step after all other processing including interpolation. If you apply final print sharpening to a 240ppi image and then the driver takes that image and interpolates it up to 360ppi, that can have a detrimental affect on your print sharpening even if the print driver uses a high-quality interpolation algorithm. And the farther your image's ppi is off from 360, the more this becomes an issue. So it's best to interpolate to the resolution the printer expects and then apply your print sharpening. Of course if you're using QImage or some other RIP that applies final print sharpening, this may not be so much of an issue. I'm not a fan of QImage's sharpening though, so for large prints I disable it and do my own sharpening.
Some people will tell you this is extra work that offers no improvement on real-world prints. In my experience, that will be true for some images which will look the same either way. Some images, the effect might be noticeable if you look really closely; and for some others (admittedly a small percentage), the difference will be more readily noticeable. To me, the easiest thing from a workflow standpoint is to always assume the worst and do the interpolation myself, rather than second-guessing the results after I've already committed paper and ink.
