As a rule of thumb I usually have a look at the number of disk drives and the number of IOPs the disk drives are attached to. Then I have a look at the type of work the disk drives are mostly performing – more reads or more writes. Most of this can be determined from performance reports.
But if you are replacing the same number of disks – 25 17Gb with 25 35Gb on the same IOPs doing the same type of work and balancing the disk drives then you should have the same performance with about double the capacity. If however the rules change you will have to do some proper planning.