CAPS OFF PLEASE !
The issue you will face, no new SKT775 support DDR2 memory,
Now the good news, Using prices from www.ebuyer.com ..
A new SKT775 DDR3 motherboard start off about £30 and 4GB DDR3-1066 from £10 or 4GB DDR3-1333 from £14. Buy the memory as a pair as most motherboards will only have two memory slots.
So a low cost solution is available, if you can't justify replacing the motherboard, cpu, memory and heatsink for a new tech.