Am planing to get iMac 27in, should i get 16GB RAM or 32GB Ram?

I mainly using Adobe CS5 – illustrator, Indesign, Photoshop for design and layout.

Presently my MacBook Pro 2.16 with 3mb Ram
programe frequent open are Illustartor, Photoshop, indesign, safari, Mail, Online streaming (movie). and the system run drop dead slow… everytime need to quit one or 2 programe…

Wondering if to run all programme mentioned above Fast, Smoothly in one go…which configure iMac is better …16GB Ram is good enough or should go for 32GB RAM
Pls advise. Thank you

Mac

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