All of these are subjective questions that you'll likely get a mix of opinions on, but here would be my thoughts, based on your use:
1) Any of the current iMacs will be good options for your usage. I wouldn't bother waiting indefinitely for updated models to be released, but I would wait through Mountain Lion's release in July, in case there's any update to the iMacs at the same time, unless you really need to buy a computer now.
2) Your usages will be fine with 512 MB of video memory, though you might feel differently 4 years from now. This is one place I'd consider upgrading now, if your primary goal is to continue using the computer happily for 4+ years, as 1GB-2GB+ will likely be standard on computers by then, so this might be the place the current iMacs would most underperform 4 years from now.
3) Get 4GB for now. You can upgrade to 8GB or 16GB down the road when you need to. RAM prices generally only go down.
4) No, I don't think you need an SSD for your usage, and the cost is still high to upgrade to one, absent a particular need. It's also possible that you could upgrade the hard drive down the road if you really wanted to. It's a much more difficult upgrade than the RAM (which is just one screw, and Apple considers end-user serviceable), but at worst you'd just pay someone a nominal fee to do the hard drive upgrade for you.
5) Monitor size is totally personal preference. But since it seems we're culturally heading toward larger and larger displays, you'll probably think the 21.5" is small in 4 years. My old 24" iMac was already seeming small by comparison to the 27", which I've quickly gotten used to.
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By the way, keep in mind that the 27" vs 21.5" is a $500 upgrade for the entry-level model of each. You also get a faster processor, larger hard drive and slightly better graphics card for the price difference, but it's worth considering whether you'd rather buy the cheapest computer you'd be happy with now, keep it for 3 years and replace it with the cheapest computer you'd be happy with then, rather than buying a more expensive computer now in hopes of being happy with it for an extra year or two. You might just end up spending more money in the long run that way.