Why are you doing this?

I guarantee you that any self-hosted system will have more downtime, and more overall management time than just sticking with Google or another provider.

I wouldn't put the e-mail server and the Web/database server on the same machine. In fact, if you're going to do this right, you probably want a mail server in a datacenter that does nothing but receive the incoming mail and hold it back in case your local e-mail server is down. And once you've done that, you might as well be using a "cloud" e-mail provider.

That said, I have used Zimbra, and it works. I will also support the recommendation of Roundcube.

When comparing Windows XP to Windows 2000, you will find that XP is faster than 2k, no matter what, as long as the reference machine has at least 128 megs of RAM.

XP boots and shuts down twice as fast. With the eye candy turned off and "classic" Explorer enabled, the interface will be far more responsive on an XP machine.

I speak from experience. My fastest computer is an 850MHz Athlon, and I use both Linux and Windows on machines as slow as 75MHz on a daily basis.