You can't check your usage on the iCloud website - you have to do this from the iCloud prefs pane, which you don't have. If you are only using it for email you would have to be storing an awful lot of messages to exceed 5GB. If you are concerned about this you can download some of them to your Mac and off iCloud.
To do this simply create a folder under 'On My Mac', go to the iCloud mailbox, select some of the messages and drag them to the folder you created. Don't try to do too many at once (say more than 100) and allow time for them to download.
If you haven't yet set Mail up to access iCloud, but are simply using the website, you can set it up as detailed here:
Entering iCloud email settings manually in Snow Leopard or Leopard
If you do decide to upgrade, the latest version of iPhoto is only £10.49, though MS Office is more of a problem. But you can go on as you are as long as your Mac holds out - Snow Leopard is a good solid system.