Joomla 1.5 is quite different from Joomla1.0.x There is a way to transfer your content so long as you use the same database for both versions, best place to check this out is on the Joomla site. I won't kid you, it's possible but it ain't that straightforward.

Then of course 90% of your components and modules in J1.0.x won't work in J1.5.x so you'll need to search for 1.5 comps and mods, again, the Joomla site is the best place to start for them.

Templates are also different, but I'm not 100% sure how compatible your existing templates are with 1.5 or if they can be converted or not.