Looking for advices building ruby-app/X-platform

i do have a project right now which should be able to work under two
OSes namely :

- Mac OS (the one i'm working on)
- Win* (no experience at all on this OS)

the project plays with audio files, extract some xml part puts that in a
data base and shows the datas on an UI.

I don't want to be X-platform at the beginning of the project but rather
make choices compatible on both platform, then i have two choices to
make :

- the GUI (fox, gtk, qt...) ?
(i would prefer fox)

- the database :
beause i'll play with xml i'd like to have an "xml" database (as it does
exists with Java : Xindice). With XQuerry...

i'd like to embed the database into the app being independent of any
default installed software.

this might appear blur to you but at this time no specification has been
writen.

also the project will be internationalized at a later time.

any advice appreciated !

PS : from my experiment in Java if, at a later time, a third OS is added
(*nix) "no problemo" because the initial project is writen on
MacOS/Unix.

···

--
une bévue