RE: Wich board?
The new board, do you want it to have more light wind range than the current Radar (125Lt)?? (it seems you're looking at boards C133/145 and even 161, but these are mostly bigger than typical for a 75kg rider would use for 5.7m - 8m range) ??
A bigger sail (in general) will give you some more bottom end/early planing. If you're often sailing in that 8+m range, then 8.7 etc could be a good advantage, but on the other hand if you are more often in (say) 7.0-7.5 conditions and only occassionally at 8.7m, the performance of the 8.7 (and board/fin etc to go with the 8.7) when used in "7m" condition wont be a big advantage !
Also worth considering how your typicalor avaerage conditions/usage will relate to the other board and sails in your quiver; by going significantly bigger in the light wind board and sail, you effectively open the gap (or reduce the practical overlap) between the big and small equipment in your quiver; if it happens that you're often spending sailing time in that "overlap" range, then from a practical perspective, it is important to NOT go too thin on coverage of that range. (rather, having sufficent - or even slightly excessive- "overlap" from - both hi and light wind equipment - minimises the criticalness and maximises the flexibilty of equipment choice around your most likely sailing conditions).. at the beach on any given day, that can be very useful..
Cheers ~ Ian