A good start is to look at what version of Tcl you've got:
info patchlevel
Then look at the contents of the tcl_platform global array:
parray tcl_platform
Now, if you are running Unix, you can obtain even more configuration information:
exec uname -a
And for Solaris users, you can find out your processor speed using the following magic incantation:
exec psrinfo -v
(the executable is located in /usr/sbin on this machine at least... :^)
On Linux, you can work out your basic memory usage profile using:
exec free
Unix systems with the luxury of a SYSV ps (like both IRIX and Solaris) can use it to discover useful info about the Tcl process itself:
exec /bin/ps -p [pid] -o {pid sz rss util pcpu time etime comm}
And there is also sysconf which is exposed on IRIX systems but not Solaris AFAICT...
exec sysconf
See Measuring your Application's CPU Utilization for a related discussion.
Please extend this page with goodies and snippets from other OSes that I know less well! DKF