Even with 512 package is ok.. cause WoW dont use alot of bandwidth.. the stability and latency is more important.. however.. usually higher bandwidth translate into better latency..

Sharing is ok.. if they are doing normal web surfing.. if they are downloading something or using BitTorrent then definitely u going to have higher latency in WoW

for a D-Link modem.. probably u can do this.. type "http://192.168.1.1/" to access the Modem setting interface.. if cant try "http://192.168.0.1/".. then click on the interface button on the top side of the page..

I forgot what its called but it should show that u are connected.. and what ur current Streamyx IP is.. I couldnt be more specific cause I am at the office right now.. and am writing this from memory..

If this doesnt help.. I'll try to write better guide when I get home later today..