I think the 27" will far and beyond handle web design.
It is the flagship atm in terms of performance and if you get the Quad Core it will handle without a problem in the world ..
Have a look at THIS article on performance and benchmarks compared to other iMacs ....
That's a pretty vague question..if you say web design involves running PS/Illustrator or other graphics programs and DreamWeaver or other authoring tools, then virtually ANY machine can do it. If you go for the i7 version of the 27" iMac, that's a quad-core CPU that will run these programs significantly faster than my 1.83 Ghz Core2Duo in my Mini..
I would love to know what "hard core" web design consists of..
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