Sounds a little overkill to me, but if the price is right you may as well future-proof a bit. Start by looking at what you have now and what kind of cpu/memory bottlenecks you are looking to fix. Look at the size of your current drive and how full it is, based on what you described I'd be very surprised if you are using more than a couple hundred G of space. 4G memory sounds plenty for what you describe, and an i5 or even an i3 with integrated HD3000 or HD4000 gfx card should be plenty. You can compare your current CPU to other i3/i5/i7 options at cpubenchmark.net.
As for the 17" screen, that's a highly personal opinion thing but since you asked for suggestions.. I've owned everything from 17" laptops to 11" netbooks. I find the laptops in the 13"-14" range the best overall size. 17" is just too big to be convenient - may as well just have a desktop. The 10"-12" size is great for mobility, but the smaller size means you either pay $$$ for "real" laptop performance or you get a cheaper laptop with lower power. Perfect if you have a desktop or larger laptop as your main computer. The 14" +/- gets you the best performance per $ and is still small enough to be convenient....
Based on what you describe (a budget was never mentioned), this is what I'd buy:
19"+ external monitor - $100-$150
wireless keyboard/mouse - ~$30
14" i5 laptop, w/4G memory, HD3000 or HD4000 integrated gfx, SSD drive (for OS) + 500G HD (for data) - $600-$800
This gets you mobility - the only reason to pick a laptop over a cheaper desktop, a nice big screen plus full keyboard/mouse for the occasional CAD/Video editing, plenty of cpu/memory for what you describe, SSD drive for the OS - this makes a HUGE difference in overall performance, and plenty of data/video storage.