Your V+ box already supports HD. You just need it connecting to a HD-ready TV using an HDMI lead, then switch it to HD mode in the Display & Audio Settings menu.
The NTL box is too old to support HD. You would need it replacing with either a TiVo (if you want to record on it), or a V-HD if you need to record. Plus an HD TV and HDMI lead of course. The V+ box is no longer available.
Click these for info on the currently-available boxes, including pricing:
http://shop.virginmedia.com/digital-.../tivo-box.html
http://shop.virginmedia.com/digital-.../v-hd-box.html
or ring Virgin on 150