If you use the 3com as the router into a giga switch I don't know whether it's presumably lower routing performance would affect transfer speeds?

However if you want to make use of your new kits routing features (Nice looking router!) you need to turn the 3com into a true modem as you say. Then you just need to plug that into the yellow internet port on your new router.
I'm using a netgear dg834v4 in it's hidden 'modem' mode plugged into the WAN port of my router which uses PPPoe to authenticate with my ISP.

Don't know if you can do that with the 3com though? I've had problems in the past with modem routers that say they have a modem mode but still do NAT etc.

If you use the 3com as the router into a giga switch I don't know whether it's presumably lower routing performance would affect transfer speeds?

The switch will only send traffic to the 3Com if it needs to, so PC A to PC B over ethernet will go via switch alone.

Modem modes can be a pain sometimes, particularly if on a PPPoA based connection, as they tend to only fully debug these modes for PPPoE mode.

Thanks all, configured the router up as suggested, 3Com can have the wireless router function turned off. Broadband speed test results about the same for the wireless so guess that suggests this is a function of the ISP connection speed rather than the router, most of the speed will be internal routing anyhow so that should make full use of the Gigabit transfer rates.