Freeview won't interfere with Sky reception. There's no real reason to get a TV set with integrated Freeview unless you want to receive a particular channel which isn't available through your Sky box. (If you buy a TV set without built-in Freeview, you can always add Freeview reception later on by buying a set top box for around �15).
When you state that you get 'most free channels' from your Sky box, do you mean just the 'free to air (FTA)channels' or the 'free to view (FTV) ones as well? FTA channels (such as BBC1, ITV1, etc) are available through a Sky box without a viewing card. FTV channels (such as Film 4) require a FreeSat card, which Sky will sell you for a one-off payment of �20 (i.e. no subscription).
The FreeSat channel line up is similar to (but not identical to) the list of channels available via Freeview. To compare the two, see here for the FreeSat channel listing:
http://www.freesatfromsky.co.uk/?pID=3
(use the scroll bar to see the list)
and here for the Freeview list:
http://www.freeview.co.uk/channels/
Chris