The Modem HAS to come form Virgin Media, and as long as you can follow instructions you could get a self-install kit from them, which is free. However if you want a technician out to come fit it for you the charge is £25.
As for routers... Best to just see if yours works happily at 20Mb first, you never know, it might do. If not i can heartily recommend the WRT54G
L model. (Note the L, as this is the best model out there). You could check if it steals speed by directly connecting to the modem with a pc run a speedtest or two, then power everything down, reconnect the router and power everything up, then do another few speedtests. Compare the speed and see if it's affecting bandwidth.
Hope this helps
