Is it worth buying a new motherboard to crossfire?

Yes but be very careful about what board you buy, I would avoid P35 and P45 boards as Crossfire performance on is poor, P35 because of the lack of bandwidth and P45 due to the poor way it handles Crossfire. Look at this benchmark comparing P45 against X48, don't be fooled by the benchmarks though it's not X8 that's the limiting factor but the poor chipset and the way Crossfire works on P45, on P55 x8/x8 works just as well as x16/x16 and that's on the latest Radeon's.

If you have an older Core 2 Duo CPU and you don't want to replace it I suggest getting the Foxconn X48 Blackops as it features x16/x16 Crossfire (as you can see from the first link x16/x16 on older boards doesn't bottleneck cards) and is currently priced at the same level as good p45 boards.