Generally speaking, in my experience, everyone tends to overestimate the quantity of resources that a website will consume.
As ever, it's a money/capacity pay-off. Unless you're expecting 30/40GB of bandwidth usage monthly from word go, I'd recommend starting off in a standard shared hosting package.
The main problem when hosting sites these days isn't really as much sustained usage, it's high-peaks caused by social media and mailouts. If you can guarantee every one of those 10,000 email recipients will click on your email, then I'd get a dedicated server.
EC2 is more flexible, but it will cost more than a dedicated solution. It only really comes into it's own price wise if you you can use it to automatically spool up and down instances as your load requires. Saying that EC2 does not handle instant load spikes at all well. If you are expecting a tweeter with 150,000+ followers to tweet a link to your site (@notch), you should either be hosting on a cluster, with caching, or have used 'siege' to hit EC2 first with a shed-load of traffic, so it spools up.
Technically speaking, a dedicated system is a lot easier to maintain and work with than EC2 - you have full control over everything. A lot in EC2 is up to amazon, and those instances can do some strange things that you'll never get to the bottom of. Be prepared to configure your ec2 instances to be totally easy and quick to throw-away; it's best to have automatic systems that detects and restarts failed instances. Whilst using a load-balancer you'll find (mostly) that everything works.
Additionally, what resources you need also depends on how much work your webserver has to do to send out the web-page to visitors. A static html site? I've seen stupidly large TV-show-driven traffic spikes coped with easily by a single core P4, 1GB RAM, Apache2-mpm-prefork machine, simply because there was absolutely no dynamic content whatsoever. Prefork because we'd not been told there was no dynamic content, php was enabled, but not used.
siege / ab (apache benchmark) are both awesome tools, that you should use to test what your site can cope with. A concurrency above 50 is highly unusual unless you're @notch'ed etc. Siege is the more advanced tool, it can pretend to be a web-visitor more completely. Running it from multiple internet connections simultaneously is best if you're using any form of load-balancer, and run it with a list of urls.
Make sure you use caching solutions - if you're using wordpress, use a caching plugin. Be prepared to get into the technicalities as deep as configuring an op-code cacher for php. Don't be afraid to ask for help, and you could always (being slightly cheeky) call in the experts :)
Disclaimer/source: I'm a techy working at http://positive-internet.com
