If you get the powerleap adapter you can upgrade to a Celeron 1.4Ghz withouth changing your motherboard. This is assuming you have a Dell computer as you said you did. I can't remember the details but go on to the dell site and go to support until you find their forums. Then go to the hardware upgrade forum and search for topics.
or check this page
http://www.roberthancock.com/dell/ . There is a link to a review of the powerleap adapter and somewhere there should be a link to buy it. It seems like a pretty good deal as it allows you to just install the adapter and not worry about changing motherboard. When it comes to upgrading again, you can just buy a new motherboard then. Or by then you'll probably want a whole new comp anyway.