Canon G2, Cokin P Series, and upgrading to DSLR

Hey,

I have a Canon G2 with the 58mm filter adapter. I am interested in
getting a square filter system so I can more effectively use the
graduated ND filters. I am looking at the possibility of getting the
Cokin P series and then another brand of actual filter. Is this a good
idea or is there something else (or additional) I should be looking at?

I would REALLY like to make sure I don't have to buy everything over
again when I upgrade to a DSLR next summer - so any comments on that
front are appreciated!

Thanks! Brian