Can/should I update from OS X 10.5.8 to Yosemite?

Is it an Intel iMac?

So we know more about it...

At the Apple Icon at top left>About this Mac, then click on More Info, then click on Hardware> and report this upto but not including the Serial#...

Hardware Overview:

Model Name: iMac

Model Identifier: iMac7,1

Processor Name: Intel Core 2 Duo

Processor Speed: 2.4 GHz

Number Of Processors: 1

Total Number Of Cores: 2

L2 Cache: 4 MB

Memory: 6 GB

Bus Speed: 800 MHz

Boot ROM Version: IM71.007A.B03

SMC Version (system): 1.21f4

You'll need to buy SL/10.6 first & get it to 10.6.8

1. You'll need to max out the RAM on your iMac for starts. Yosemite works well with lots of RAM, and not so well with not so much.

2. As said, you'll need to buy Snow Leopard from an Apple Store ($20), install that, then download Yosemite from the Mac App Store (free).

3. If you're running Leopard, then that's a 7 or 8-year-old Mac. Why the sudden urge to upgrade? The chances are that many of your third-party apps won't work in Yosemite and will need updates or alternative (additional expense). If your Mac does what you need at the moment, then perhaps it's best to keep it as it is.

You can expect your hard drive to fail at some stage soon: you might want to replace it with a fast SSD.

Alternatively, you could sell this Mac on eBay and put the proceeds towards a new or newer model.