With SIP you only need a small range of ports forwared to the phone(SIP 5080, RTP 5004, and a few up in the high range for the voice channels).

There is no direct interface that I've found from H.323 to SIP, but there is Asterisk PBX which handles SIP natively, and a module that'll interface H.323 to Asterisk making a SIP<->H.323 gateway possible.

Asterisk PBX Software
http://www.asterisk.org/

H.323 module for Asterisk.
http://www.inaccessnetworks.com/projects/asterisk-oh323

If you want to test a direct IP call from a SIP phone, I can help out there, just PM or email me direct.