Ok, you have Leopard which is the latest Mac OS release. AFAIK, they are up to 10.5.5 at the moment.
So you can buy the disc for Leopard, either from
Apple, maybe even
Amazon or an auction site like eBay ... just make sure you pick up a
'Full Retail Version' which is basically a universal install that will work on all Macs that meet the Leopard Minimum System Requirements
(which your mac must already meet, of course ):
- Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
- 512MB of physical RAM
- DVD drive for installation
You could also decide to go for the previous OS version; Tiger, which you may be able to pick up a bit cheaper from the same sources as above
(except the Apple Store)
Whatever you decide on, make sure you read the fine print very carefully and that you are buying a Full Retail Version
(Universal Install) as some auction sellers can claim this without realising that the disc/s they're selling require a *certain* version of Mac OS to *already* be installed on the Mac. This type of Installation disc/s is called an
"Upgrade" set and I'm sure there are plenty of people who have mistakenly fallen for this ....