Looks like I'll have to look into a new one. Problem I have is that the new ones can't be DIY upgraded in terms of SSD and ram as I understand it...
Depends what you were to get. RAM on 27" iMac's is still user upgradeable.
Quite an expensive purchase (to get videos from this action cam to run smoothly)...if everything else you do on this 2009 iMac is fine.
If it were me. Before purchasing a new/newer computer. I would try to find a friend, family member, coworker, etc who has a newer Apple computer...and see if your videos play smoothly with a newer Mac. Or maybe go to your nearest Apple store...and see if they will let you play your videos on a new computer.
Word of warning. If you go to the Apple store...you are VERY likely to walk out with a new computer (needed or not)!!!
...and the cost from Apple is eye watering.
Not exactly true. The price of your entry level Late 2009 21.5" iMac back in 2009 was $1199. The price of a brand new 2015 entry level 21.5" iMac is $1099 ($100 less than what your 2009 iMac cost). And even the better one level up 2015 21.5" iMac is $1299 (only $100 more than what you paid for your 2009 iMac).
These prices are not "eye watering" compared to the price of a 2009 21.5" iMac.
And if you purchased the "better" $1299 2015 21.5" iMac I mentioned above (refurbished)...it would only cost $1099. This would be the exact same price as you paid for your 2009 21.5" iMac...and...it's a better computer in the product line (better than entry level)...which is what your 2009 21.5" iMac was (entry level).
http://www.apple.com/shop/product/FK442LL/A/refurbished-215-inch-imac-28ghz-quad-core-intel-core-i5
- Nick