Most likely you'll have to buy the latest version (8.0).
This allows you to use the older version. I had to do this a few months ago because NI doesn't have the 6.1 App builder any more.
As far as I know, there were never any large price differences between the different versions anyway, so it shouldn't cost more.
Hope this helps
Shane.
Using LV 6.1 and 8.2.1 on W2k (SP4) and WXP (SP2)