My mother in law has Now Wireless Broadband <www.mynow.co.uk> at home, and I
was wondering what is the best Wireless Router to use with it? At the moment
it is connected to the PC upstairs by USB.

I was thinking along the lines of getting a DLink DI-524 Wireless Router.

not sure if this is a serious question, or an advert for an isp which i've
never heard of, and wouldnt touch with yours.
if she has no phone line in the house, and limited internet needs then this
service may be ok, otherwise i'd take advantage of the 30day trial and get
shut of it, much faster available for same or less money, and with clearer
FUPs

as to routers, the dlink is ok, but check that the supplied modem has
ethernet out, if just USB then your buggered.