Yes, I know it would be cheaper to use RAM you already own...but since you now have a new motherboard which is designed to go faster, why not take advantage of it?
And more importantly, because the specs of the motherboard indicate a different type of RAM, using anything other than the specified RAM is taking a chance of damaging the newly purchased MOBO. That would also lead the more money spent for a replacement. Personally, I choose to follow the specs indicated.
Hope this helps.
I am planning on purchasing a Gigabyte GA-P31-ES3G iP31 motherboard, which a website lists as compatible with PC2-6400, PC2-5300 and PC2-8500 ram. Would it be possible to use the two 2GB PC2-4200 modules I already have with this motherboard?