Well, how ironic! I've just been asking myself these very same questions.
I have looked at these two:
www.ascent.co.nz/productspecification.aspx?ItemID=372080 (unable to find any reviews)
www.ascent.co.nz/productspecification.aspx?ItemID=364368 (reviews seem to indicate that its not that great a performer in the wireless area)
My preference, finances permitting (maybe my wife will buy me one for my 40th!), would be to stick with my current ADSL 2+ router (a Dynalink RTA1025W which is pretty good), and disable it's on-board Wireless G and plug a Wireless N router in to replace it. (thereby just using the Dynalink as purely an ADSL router with no wireless functionality)
The newer model Airport Extreme appeals due to its dual band wireless support which as I read it means that Wireless G devices will not affect the performance of the Wireless N devices (if you run them both the performance of N is normally severely degraded as soon as any G devices connect to the wireless infrastructure).
The other advantage is that it support 5GHz wireless N (with compatible clients, of which the iMac should be one) and as this is outside the already over-crowded 2.4GHz spectrum higher throughput can be obtained due to substantially less interference.
The above is based on my understandings as listed, if anyone would care to correct me or offer any further advise I would appreciate it.
Cheers,
Jamie