Best VPN server software for low CPU useage

Hi could someone kindly advise me on which is the best VPN client
between FreeSwan and OpenVPN in terms of processing power on my small
server.

PPTP is as im told pretty much insecure, however if it i can run many
more clients at high speed on my remote server then i will certainly
consider it (as long as it is still reasonably hard to eavesdrop -
i.e. will take 4 to 5 hours of time rather than fast hacks like WEP).

I may have upto 30 VPN clients and i am concerned this number of
clients will be slow with intensive encryption/decryption of traffic,
will i need to consider upgrading when i hit this number or will
things slow down before this (current server is 384MB ram and probably
around 750MHz as its on a VPS)?

Thanks in advance