Alan,
to CF.
1. Correct - ADSL and Cable broadband are different technologies.
4. The existing USB wireless adapters will work fine with a new cable router. It's only the connection between the router and Virgin that is different.
5. Correct again - the main reason for offering high speeds in a home environment is for transfers within the home network, eg sharing large media files with other PC's in the home, streaming media to an XBox, backing up across the network. If you do none of the above access to the interenet will be limited to 2Mbps (shared among all the users).
3. If you have all Belkin adapters currently, I would stick with a standard Belkin G router. Typically, anything faster than standard 802.11G (54G) only works when all adapters and the router are from the same stable. eg A belkin 54G+ router and Linksys 54GS card will only talk at standard 54G speeds. Can't comment on the specified routers as I have a Linksys WRT54G myself.
HTH.
---------- Post added at 13:43 ---------- Previous post was at 13:38 ----------
Something like
this would do fine, although again I have not tried it myself.