How can I upgrade my mac os x 10.5.8 to mountain lion

http://www.apple.com/osx/how-to-upgrade/

You need to first upgrade to 10.6.8, however the retail SL disk is no longer sold by Apple online. You can try calling AppleCare, look in an Apple store or look on Amazon.

Make sure you meet the requirements:

http://www.apple.com/osx/specs/

No PPC apps will work, nor are they supported.

Upgrading to Mountain Lion - http://www.apple.com/osx/how-to-upgrade

Mac OS X: System requirements for Mountain Lion (10.8) - http://www.apple.com/macosx/specs.html

Lion and Mountain Lion application compatibility - http://roaringapps.com/apps:table

The tricky part will be getting Snow Leopard first. You can try phoning Apple Support (not sales) and see if they can get you a copy, otherwise you'll have to look on places such as Amazon or ebay where I see prices are already rising.

Early 2008 is a 4 year old machine and at it's end of life stage.

Since 10.7 and 10.8 require a lot of new software purchases, 10.6 will run most of your software with a update, you might want to forgo the Mountain Kitty for a new machine with all the hardware attributes that come with it.

Do upfgrade to 10.6.3 then to 10.6.8 via Software Update, swtich your browser to Firefox or Chrome as it gets updates (Safari 6 for 10.7 or 10.8 only)

When the wheels fall off that machine, then buy a new 10.8 or 10.9 one.

Sometimes it makes sense to upgrade, however with a four year old machine caked with dust and a near dead video card, your better off milking it and saving your money for a new machine.

If your 10.6.3 upgrade slows down the machine, then backup your files and do a fresh install, that will make it fast.

10.6 is faster than 10.5 9better video drivers). 10.7 is slower than 10.6 (can't buy it now anyway), 10.8 is faster than 10.7 (however only if your hardware is recent and above the minimum specifications for it)

How to erase and install Snow Leopard 10.6

FYI for all, I'm currently running 10.5.8 on a late 2008 Macbook Pro 15" 2.53 GHz. I just called Apple Care and they set me up with Snow Leopard for $29.99. He told me that once I've installed it, go immediately to the Apple, click "Software Update," do the update and restart the computer. Hope this helps!

how'd that end up going. i'm running on the same unit and am looking to update the software

I also have a computer I'm looking to update. It's 10.5.8. Do I still have to download 10.6.8 first? If so, is that free? Also, if I do end up getting Mountain Lion, will it slow down my Mac?

You have of course verified that your Mac can be upgraded? You can do that here:

https://discussions.apple.com/docs/DOC-3761

And perhaps you can tell us why you want to upgrade.

I'm able to update. Um, I want to be able to install newer applications on it... 😕 Why? Anyway, my main question is, will it slow it down?

I did not experience a slow down, if anything the opposite, when I upgraded a Macbook 7.1 from 10.6 to 10.8. That was some months ago. This is my personal experience.

Your other question: Yes, you need 10.6, it is a purchase from Apple ($20) on DVD, not a download.