Unless you're technically inclined and willing to void the warranty. I wouldn't do it.
Whether or not you'll miss it - how much music do you have on your computer, do you have videos? Do you want it all or most of it with you? Do you want to do back up file storage. Take all of these into consideration, then think of what you might be downloading/buying for a couple of years into the future (during the lifetime of player, be it iPod or Creative).
Would the extra 20 GB be worth it to you to be locked into iTunes?
to 80gb if Creative comes out with one. Or is it even possible to upgrade hard drives on MP3 players. Also, I am deciding between the Creative and the Ipod 80gb, if I go with the Creative, will I really "miss" the 20gb of storage space had with the Ipod?