ok, thanks for up 2 for the info, i'll surely use PNG and .x then :p
from what you said, .x supports textures well, right ?
about GARTSize, my Xorg.0.log has several values :
(II) RADEON(0): Using 8 MB GART aperture
(II) RADEON(0): Using 1 MB for the ring buffer
(II) RADEON(0): Using 2 MB for vertex/indirect buffers
(II) RADEON(0): Using 5 MB for GART textures
i don't where to add the options in xorg.conf though, as i don't have them by default.... i guess it is within the device section of the ATI card but i'd like confirmation before screwing things up

(or i'll test this evening)
The game, if finished one day, will be of course heavily moddable and open source

(or at worst, free...)
edit : really important question : is it possible to put the image we want for GUI elements ?? (with transparency if the image is transparent, and etc.)
or are GUI elements/windows/... only grey ? (a friend told me the buttons could be changed with an image, but no other GUI elements are customisable like that..... which would require him and me to redefine our own GUI :/)