An 802.11g wifi card and router can transfer data at a theoretical maximum speed of 54Mbps or half the speed of a wired connection. An 802.11n wifi card and router can transfer data at a theoretical speed of over 100Mbps or equivalent to a wired connection.
Connection with the slower 802.11b/g wifi is fine for most purposes like browsing the web, but if your wife intends to stream video over wifi then the slower speed may be a problem.
802.11n routers are more expensive than the 802.11b/g ones but unless you have, or intend to buy a laptop with an 802.11n card fitted there isn't much point in spending the extra money, you won't get to use the speed. I myself have a D-Link DIR 825 draft n router and have had no problems with it, you can pick one up for around £100 or so, a draft b/g router can be found for around £50 or so.
If you have draft n equipment then that would be the way to go otherwise save the money and pick a draft b/g, all routers usually come with 4 ports to connect via wire along with the wifi capability. As your provider (like my own) is Virgin you will also have to make sure the router is for cable and not NDSL which is used for BT broadband.