How long is a piece of string? But seriously, what kind of applications are you likely to be developing for this server? What sort of demand can you see it having? As you've said you'll be developing SQL-related stuff will you be using MSSQL on the same server, the desktop edition, or some other SQL technology (such as MySQL)?

As for a hardware spec, you shouldn't go too far wrong with a Windows 2003 web server with 1Gb RAM, 80Gb hard drive and at least a Pentium 4 2.8Ghz CPU if the applications you're writing are nothing too complicated. This would likely require much more oomph if you're intending to put MSSQL on it. Bandwidth is a bit more difficult to gauge at this stage.

To give you some idea. I got away quite happily running Windows 2003 on a Celeron 2.8Ghz box with 1Gb RAM and running a mail server (MDaemon), couple of small IIS web sites with pages generated from a MySQL database and Perl. MDaemon also had it's own webmail server which ran from IIS. Perl worked, but performance wasn't that great, of course, but it did me quite well for the small amount of use I wanted from it.

Regards,

Martyn