I think your iMac has 8 cores (4 physical, but 8 with hyper threading), so 16 gb would probably be enough... But if $200 won't break you, I think I might just go ahead max it out (yes, it can use 32gb)
As far as paying someone... First check out an installation video:
http://eshop.macsales.com/installvideos/
I think most users can handle this upgrade
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW