RAM or random access memory, is your computers short term memory also know as the main memory processes happen from here so less RAM less performance more Ram better performance depending with your processor.
DDR refers to Double Data Rate , with the introduction of this technology it is important to note that DDR2 2 GIG 800MHZ RAM and DDR3 2 GIG 1333MHZ are very different in performance , not only that but the DDR3 is better knowing the difference is is key.
To tackle tot issue
With the same hardware that means same processor and also same power supply and same everything except the ram slots ,
using a G41 +ICH8 , 775 socket
It supports both DDR2 and DDR3 with everything same you will find that the performance is the same .
the question is why ..?
The reason is simple , even with the increased band width the computer does not have enough workload to put the DDR3 RAM to full capacity and so the performance enhancement is not noticed.
The only notable difference can only be noticed if you use benchmark software that will or at least try to maximise performance and also when you use servers that have a lot of work load in this case the difference is clearly noted ..
But for a normal gaming computer if you will, it is a matter of preference and socket constraints of the motherboard, because some don't allow both types of RAM slots .