The am2+ CPU has been superceded so you may have to find what is left out there for sale. I suggest newegg.com.
But these are likely to be REPLACEMENTS rather than upgrades.
Besides I've rarely found any motherboard replacement to cause the system to work that much better.
Would like to know what is a suitable upgrade for asus m3n-hd/hdmi nforce 750a sli socket am2+ mb, without any other changes, just motherboard.