Both methods use encryption, Logmein uses HTTPS, and the built in VPN for 2003 uses IPSec(how strong is defined by you in IPSec Policies) so long as you have it enabled, you will notice degraded performance for IPSec on the VPN side if you don't have a separate NIC. An onboard NIC will consume CPU cycles to peform encryption while an offboard NIC should handle its own encryption of data.
"I'm certifiable, not certified. It just means my answers are from experience...not a book