<roll eyes>
If the banquet is so cheap that e-mail is used to send the invites
then the fare is probably just as cheap.  Really expecting a high
percentage of replies to an *e-mail* for a banquet?

Since you used the term "evite", why aren't you using evite.com?  They
are an IAC Company member which means when you signup that you
establish a business relationship with the 60 brands of IAC who can
then spam you legally from all their affiliate companies, their
partners, or other 3rd parties (go read their so-called "privacy"
policy).  I suspect they may also spam whomever answers your evites.
Since the maker of Gator spyware (changed name to Claria) invested in
evite (http://www.benedelman.org/spyware/investors/), well, you decide
if you want to trust them not to install spyware on your host.

http://www.pcworld.com/article/id,136152-page,1/article.html
http://www.scurrilous.com/blog/archives/2007/03/30/evite-alternatives/

There are probably better services.  Google around
(http://www.google.com/search?q=%2bemail+%2bparty+%2binvitations, or
pick your own search criteria).

Outlook is a PIM that includes e-mail.  It has meeting management
where you can send out invites and the recipients can accept and even
add it to their calendars but I doubt your 1000 invitees are all using
the same Exchange server.