This is a very common problem. The thing to remember is if you have a 10.6 compatible Mac, you do NOT need to purchase 10.5. If you don't have a 10.6 compatible Mac, you can buy 10.5 from Apple Retail. 10.6 compatible Macs are listed here:
http://discussions.apple.com/thread.jspa?threadID=2126451
And 10.6 is only $29.95 for people who have 10.5 or 10.4 and are compatible.
Mind you, you may need to upgrade the RAM to 1 GB, or increase your hard disk space to 9GB + 15% (an arbitrary amount free that works best).
For 10.5, you need 15 GB + 15% (an arbitrary amount), and 512MB of RAM or more. 10.5 is available for around $129.95 if you buy from the Apple Store phone sales (not available on the website).
Alternatively, if your Mac is compatible with neither, or you can't get it from Apple Store Phone sales, 10.5 is available from numerous used and refurbished Mac shops*:
http://www.macmaps.com/usedrefurbished.html
Note: 10.5 is not easy to upgrade to if you have 800 Mhz or less on your Mac. 1 Ghz is more.
10.5 does not offer Classic compatibility, though some Macs can boot into 9 and 10.5:
http://discussions.apple.com/thread.jspa?threadID=2176255&tstart=10
PowerPC Macs also do not get Boot Camp compatibility either!
Make sure though your 10.5 installer disc looks like

* and does not say DropIn, OEM, or Upgrade.
Before upgrading, make sure your data is backed up at least twice*:
http://www.macmaps.com/backup.html
and the backup is not connected to the machine during the upgrade.
You do not need to erase your system, as long as it is healthy, and not portraying any software issues. If there are software issues, be sure to ask what software issues need to be fixed here:
http://discussions.apple.com/forum.jspa?forumID=758
and posting a new topic there.
Once your system is ready, then make sure your software and hardware is compatible with the operating system you are going to:
For 10.5 Intel Macs (same as Snow Leopard compatible Macs, except you can't afford 10.6 compatible software):
http://tinyurl.com/cnet105Intel
http://tinyurl.com/cnet105PPC
http://support.apple.com/kb/HT1370
For 10.6:
http://tinyurl.com/cnetsnow
http://snowleopard.wikidot.com/
http://www.macintouch.com/specialreports/snowleopard/slcompat.html
And remember what I said about backing up.
- * Links to my pages may give me compensation.
Message was edited by: a brody