Question about Red Hat Linux Enterprise

I have a question about Red Hat Linux Enterprise. My experience is
almost exclusively in Windows and I know almost nothing about Linux.
Here is my situation at work: we have a web server running Apache with
a MySQL database, but the site we have is running Windows 2000 Server
on an old HP box. My boss is going to get a new machine, but she
wants Linux put on it as the OS instead of Windows. The site gets
about 2.6 million hits a month and the server has 2 gig of
downloadable files on it. Here are my questions:

1. I'm trying to decide between putting Red Hat Enterprise Linux or
Red Hat Advanced Platform. Which software do you think would be best
for the type of website we have? There is no backup server or any
clustering or anything like that.

2. I know most versions of Linux are free, but you have to pay for
some. When I went to the Red Hat site, it showed difference
subscription prices, which are support for the software, so does that
price include the software itself (which I assume they'll send to you
with documentation) and the support or what?

3. What kind of machine should we get to run the versions of Linux
above (processor speed, RAM, etc.)?

Thanks. Sorry if these are basic questions, but I'm new to Linux. If
you need anything clarified, just ask. Thanks.