I just installed it on my 800 yesterday.

As for the config, you'll want to get your *.pcf file from your existing client and convert it using the perl script from the vpnc site. Grab the vpnc-0.3.3.tar.gz and extract pcf2vpnc. You'll need perl installed. On Windows you can get it from ActiveState for free.

This will generate the vpn.conf by using a command similar to
perl pcf2vpnc vpn.pcf > vpn.conf

Connect your n800 to your PC and copy the file over to the SD card. Then, you'll want to open a terminal and copy it over to the right location.

(mmc1 is internal, mmc2 is external)

sudo su -
cp /media/mmc1/vpn.conf /etc/vpnc

and run it.
/usr/sbin/vpnc /etc/vpnc/vpn.conf

I just tested mine out tonight, however, our network requires a firewall to be installed that apparently isn't supported in vpnc yet. Version 0.3.3 is what I have on my Nokia.