( partitions their sizes and space usage )
Do note down on what hda
? lives what partition, this might come in handy at re-install, repartition and crash recovery.
( hda is the first HD, hdb the second, hdc the third etc. etc. hda1 is the first primary partition, hda5 the first logical partition on the first HD )
Everything in one partition is not a good idea, at least you should have a separate /home partition and give it space enough to grow. The clever thing about a /home partition is that when you do a re-install or upgrade you can leave it as it is, saving all your personal settings, mail, addressbook etc.
The minimum number of partitions and their size are ( if you have 5G to spare and full install of modern distro ) :
3 G for /
1+ G for the /home partition
500 MB for the /swap ( more swap is waste )
( Sure you can do with a lot less, some distro's are happy with -500MB and still have X )
A more comfortable partition table is ( 10G to spare ):
5 G for /
4+ G for /home
500 MB for /swap
A "deluxe" and more complicated partition table is ( 15+ to spare ):
2 G for /
4 G for /usr
1 G for /var ( a lot of writing is done in the /var/log's )
500 MB for /swap
5+ G for /home
7+ G for /backup ( storage )
Any extra space could be added to /home and /backup.

Bruno
Thanks to LilBambi
NOTE: An important note from Prelude76: