I’m not familiar with iChat. It’s using .Mac network? And Spark is primary jabber (XMPP) client. Though it’s capable to connect to other networks (AIM, ICQ, MSN, etc.) through a IM Gateway plugin, which should be installed on the Openfire server, where Spark client is logged in. It’s not multiprotocol client. I dont remember IM Gateway to support .Mac, but iChat can talk to AIM users. So maybe it’s possible to talk with iChat user with Spark logged to AIM network. But i suppose you are not willing to install a server to do that. There are also public servers, e.g. igniterealtime.org . But i wasnt able to login to any other networks there. For video chats one can use Red5 plugin (server plugin again, and still beta), though it looks complicated to me.
In overall it seems that Spark is not a good choice for your needs.