Until today I always read that mysql isn't possible yet, cause the wl500gx just doesn't have enough RAM and/or CPU power. So that would be a NO.
But ... I >HAVE< managed to get it up and running, I installed a chrooted debian sarge (I used info from several tutorials here in the forums) and simply installed a apache2 with php and mysql using apt get. BUT I never tested how the router would act if I really used that, had no time yet. All I can tell is that I needed some swap space cause mem usage of course increased heavily. I think there are more "efficient" solutions at least for the php enabled webserver, maybe with only running the mysql under the chrooted debian.
Oh and a 256MB Stick is PROBABLY enough even for the chrooted debian, BUT ... you'll be near the limit. I for myself won't be testing this again before I manage to get an external 2,5" USB harddisk. (used a 512MB Stick so far)