Well, I don't agree with Daniel here [I usually do :-)) ]

I have a feeling that, for a Firebird dedicated server configuration, XP
Pro will be exactly the same that a server version.
I think it will be better, just because the server versions usually
turns on by default some services that you wont need for Firebird, and
that will be hard to turn off. :-)

Anyway, a Firebird server uses mainly TCP/IP, RAM and Hard Disk access.
I don't expect too much improvements in this area for a server version,
that could influence Firebird. Can anyone explain to me why a server
version will be better to Firebird ?

But this is another discussion: Kevin NEEDS to buy a server version.
The problem is with EULA. Microsoft don't allow more than (5 or 10, can
be sure) connections in the EULA. Just pick up a XP Pro CD and look at it.
But it's still possible in an technical way. :-)

One of our clients used a W2K Pro for sometime with 32 connections,
before someone looked at EULA. They switch to Linux.
Yes, there are times that Microsoft help our clients to achieve better
solutions. :-)

Artur