After trying (a lot of) things suggered by different users I've just found one fixing the problem for me :
Going to Documents&Setting/(user)/Application Data*/Mount&Blade and deleting options.dat and rgl_config.
After doing that and relaunching the game I can enter options, and the game seem to work normally (I can at least enter taverns and battles without freezing, even if the warnings are still there in the rgl log).
I've also noted, the vertex buffer part of the log is different :
"vdt_regular :Out of Static vertex buffer memory. Needed = 492923 Available: 200000
Expanding Buffer Capacity to 616153 " (now)
instead of
" Needed = 205248 Available: 200000
Expanding Buffer Capacity to 256560
begin_setup end_setup begin_setup end_setup begin_setup end_setup " (before I deleted options.dat)
So I imagine, before I deleted option files, they bugged the program so it wasn't fulfilling the real M&B need of buffer capacity.
* the folder may be invisible in windows xp familial edition, so may need activate "see invisible files"