Drupal Conversion to New Servers: Win or linux

I have been Contracted to convert an existing website to Drupal. I was hired as the developer but the Company is asking me to help size the servers that will be needed.

They are having their complete network redone by Symquest and have no real IT staff. I have been a java developer for some time but no experience with Drupal, or PHP until now. I am not an infrastructure guy and any sites I have familiarity with are much bigger than this will be. They seem to want to serve from Windows server VMs which I am not accustomed to but not against. I feel better about AIX or linux but it's not my site.
I have a fairly good understanding of caching at the apache level to keep down number of requests for ancillaries such as javascript, css, and images and that kinda thing.

Anyway any advice anyone has would be definitely appreciated. Here is a bit about the implementation. I'd be happy to provide more info if needed.

- Site will contain almost exclusively content (unfortunately plenty of ads on every page), not much in the way of file or image upload/download. No ecommerce. Will maintain a somewhat active forum. High expected use of comments and ratings.
- Only a few custom modules and mostly for new content types. Will be writing my own theme.
- Currently about 30k visitors a month but I would like to size for about twice that which I estimate to work out to about 600-750 concurrent users max. I actually haven't seen the data yet so a guess of 500k users.
- I would hope to keep to inside 5-second page generation times at peak.
- I do expect to do some db optimization, certainly keeping the data lean and query caching.

If windows server, any OS preferred? If linux?
Also, Total RAM and CPU recommendations would be great.
Load Balancer necessary?
How necessary would a seperate DB server be?
Also, is InnoDB necessary at this level of activity?

Thanks for reading. This is my first post, just got introduced to Drupal in the past three weeks and have been extermely impressed with both the product and the community.