[iMac] Newbie with viewing distance question

[M.Milligan]

Hello all. I'm new to this list. I've used Mac notebooks for many years 
now. Soon we're buying an iMac.

Our concern is the viewing distance required for the 20" & 24" screens. 
Is there a minimum viewing distance one should be from the screen?

I want to use the iMac for home computing, web browsing & email and 
also for Photo Shop work on my digpix.

Should I max out the ram to 4GB? Is a 20" 2.4GHz clock speed suitable 
for digital photo correction and related work, or is there that much of 
a difference with the 2.8GHz?

Any help is greatfully accepted. I thank you all in advance. Cheers.

Murray