Installing Varnish is really simple and easy to perform, and is very unlikely to give you problems with your server that any other more complex methods could give you, like using light servers or so. Probably you cannot compare both performances, but to start improving you Drupal server is a good point from where start.
1. Install Varnish. In Ubuntu / Debian is as easy as:
sudo apt-get install varnish
2. Install Varnish module in your Drupal:
Be sure not to make the START=no mistake.
3. Start your Varnish server: sudo /etc/init.d/varnish start
Go to admin/settings/varnish in your Drupal and select enabled.
4. Copy the Varnish Control Key which you´ll find in /etc/varnish/secret (you'll need to be root or read with "sudo vim /etc/varnish/secret").
5. Enjoy :-)