Hi should update to 10.4.11.
If you both have Macs you should be able to use iChat.
There is one version of Skype (v.5 or later) that is for Intel Macs only.
The version for PPC Macs is v. 220.127.116.116 which you get from this link:
Just scroll down until you find it, or possibly this direct link will get you there a bit quicker: