You might want to post your location, as you might get lucky and find someone in your area who posts on the forum. They will be able to exactly what your area is like, as performance does seam to vary from area to area.
If you do take out the 20Meg service and find that it doesn't live up to spec. Just remember with virgin media you have a
money back guarantee
And as mentioned earlier, you will be better off getting two or even three services. As it will probably work out cheaper.
Triple builder
XL broadband, M TV and M Phone for £ 31 /month (£20/month + phone line @ £ 11/month) + Join Online, Save more. Add TV and phone to get a whopping £50 credit!
and stand alone XL broadband is £ 35 / month after initial discount
ooppps and yes they'll come out and install everything for ya!