1. Yes, it will kick total rear for those types of tasks. I have a 2GHz core 2 duo macbook (same ram speed and stuff) and it is great, especially since snow leopard, with how fast this processor crunches numbers. Yours will be more than 1GHz faster than mine... AWESOME. Also, OS X uses RAM much more efficiently than *Ahem* some other operating systems that are out on the market. I have 4GB of DDR3 in my macbook, and after 4 days of uptime today, I finally got my first page out (as in my system had to use some virtual memory for a second, yikes!). That was after some fairly intensive Garage Band sessions where I was running about 20 tracks side by side with layered effects. My dad just got a 20 inch iMac with the same specs as my laptop from the college he works for, and he loves it. I imagine the 21.5in with better specs would also be extremely pleasing.
2. Yes, if you're very very good at being careful. On these new ones to get at stuff, I think you have to take the screen off. Granted, 4GB of RAM is enough for most of what you'll be doing, I'm sure, but if you want more, I'd try to get it preinstalled. Apple isn't charging any more than you'd be paying for the RAM from Crucial right now, and they'll install it for you. Go to 8GB now if you think you'll want it in that machine. HD, 500GB is huge man, and network storage is becoming more and more common, so you can have a lot of your stuff on a network drive accessible by all computers.
3. I don't own it, but I want to. (I actually want an i7 or i9 iMac, if they come out with an i9 one in the next generation or 2). My next computer will be an iMac. Well, next computer probably an iPad (also second gen)... next desktop, an iMac.
4. Yea, I need to buy it while I still can for this MacBook.