How I resized my array, based on a Promise SX4000:
Replace one drive at a time. The array recognized that a drive was "missing" and began rebuilding the data. It took about a day per drive to go from 160GB to 200GB drives (switched from Hitachi's time bombs to Seagate drives).
Once the rebuilding is done, there was a total of 120GB of extra space (4 drives in the array). I used Acronis DiskDirector to change the sizes of my partitions to fill the extra space.
The rebuilding process depends on your controller.
So yes, assuming that your array is supported, you should be able to resize your partitions after the array has finished assimilating the new drives.