What do you mean? As far as I know a dvd has 480 interlaced lines of resolution, you only see 240 lines at a time on the screen. A "doubler" converts the 480i (interlaced) to 480p (progressive), so you will see 480 lines on the screen at a time.

A scaler will take those 480 interlaced lines and convert them to however many progressive lines you want. I use an HTPC at 600p for anmorphic, and 768p for 4:3 images.

the iscan pro is fixed at either 480p or 540p (540p is similar to 1080i)

None of these methods increases resolution. They just increase the perceived resolution- doublers only do 480p, scalers do 480p and beyond. Line doubles are rarely used in high end systems because higher resolutions are needed to eleminate scan lines on high end projectors.

~Jay