you have basiclly around 2Gb of space for installation (apps/games n etc) on you n900...the easy way to find how much you got left is by installing FAM (faster application manager) cause this apps will automatic show you how much space left in...

1.opt
2.rootfs

now if you unable to install FAM due to not enough space then try using xterm & type....

root
apt-get autoclean
apt-get autoremove

this command is use to clean & free some space by removing useless files that have been left by ordinary HAM...