Well the other thing those buffers effect is the number if tracks that can play back at once. So rather than massively reduce them to alter you latency you could just record stuff and re-align it after. This also means that you don't have to spring for a new computer.

As far as fire wire vs sound card, well it depends on what you want to do an how good the ADCs are in what ever you are using. Either way I don't think the problem is with the interface but rather with the system / software / its configuration.

However springing for some more RAM is never a bad idea