Splitting up your filesystem layout gives you far more flexibility when installing software, prevents your entire system from crapping out if a rogue application fills up a partition, and makes fsck faster during a forced reboot
The scheme I use is:
/ 495MB
/download 1011MB
/home 3.1GB
/opt 3.4GB
/tmp 695MB
/usr 7GB
I have a 10GB NTFS partition at the head of the disk and a 594MB shared FAT32 partition right next to it. Over time I intend to shrink the NTFS partition (I hardly ever use XP anymore), and give the space to my Linux partitions.