Question : Problem: How much ram should I purchase for this computer?

My son would like more memory for his computer.  I would like to purchase some as a gift but am unsure as to what to get.  He has a AX4sPE Max 11 motherboard with 4 slots for memory.  Currently he has 3-256 mb sticks of ram.  If i purchase 2-512 mb sticks will it function correctly?  Then he could have 2-256 sticks and 2-512 sticks for a total of 1512.  I have read somewhere something about if you mix different amounts the higher amounts will only work as fast as the slower amounts.  I don't want to waste my money if the 512's will only work as fast as the 256's.  Any advice would be welcome from this newbie. I am assigning this a high point value because I need to order this ASAP to get it for Xmas. Thanks

Answer : Problem: How much ram should I purchase for this computer?

Be sure to buy dual channel sets as they are matched sets. Do not buy individual sticks of memory.