I would like to have a real time visual status screen displaying the current CPU usage (and possibly memory consumption) for a number of remote machines. These machines will be reachable only using TCP (not UDP etc) and the graphical client will need to run on Windows. Does anyone know of such a tool?
Cacti, Opsview, SCOM, and plenty of other pieces of monitoring software can do this and will display the results in a web browser or Windows console. Keep in mind that even at 1 minute sample intervals, you'll be creating a lot of traffic and causing additional load processing the SNMP/WMI requests on the boxes being monitored. The "standard" value for samples like this are 5 minutes. You can absolutely set them to be more frequent, but be cautious.
You might look into nagios http://www.nagios.org (there is both a free and paid support versions).
You can try to use Nagios. If you need notification when a some service or host become unavailable i think that Nagios is best solutions.
If the collection server is Windows you can also look at Spiceworks. Also free.