DDR2-800 5300 should be DDR2-667 5300. They could most likely be replaced with DDR2-800 modules, but you should make sure. It depends on the system/motherboard, but you didn't provide that valuable info.
DDR2800 will run fine at 667 speed if it's needed.
Shouldn't be any problems, unless it needs to be registered ecc RAM or something like that (not normal for desktops).
------------------------------Intel E8500 - 4.26Ghz - 533 x 8 - on air cooling with DDR2-1066 running native
Sapphire Ati HD4850
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