|Upgradation? is that a word? lol|
DDR2-533 is the correct RAM to be running with an 800MHz FSB Intel CPU. If you're adding the DDR2-667 to the DDR2-533, it will all run as DDR2-533 anyway. If you removed the DDR2-533 & replaced it with DDR2-667, the performance difference will be minor, if at all, unless you increased the amount of RAM.