I have a 1525 and years ago when 4GB sticks first dropped below US$20 used, I brought it to the computer store and tried every combination of 8GB DDR2-800 they had, and none would POST. One 4GB stick and one 2GB stick (6GB total) always worked fine, at 667MHz. Strangely it reports Dual-Channel not Flex despite 200-pin soDIMMs being same 64-bits wide as regular DIMMs--so "should" be half-bandwidth after 4GB. Oh well, half-bandwidth still beats paging to disk.
Officially, the GM965 Crestline chipset only supports 4GB PC2-5300 DDR2-667 but as I said, unsupported 6GB worked fine at full speed.
The chipset supports 800MHz FSB processors but memory only up to 667MHz, so 1066 memory would be a waste.
It wasn't until Cantiga chipset such as GM45 were DDR2-800 speeds supported, and many of those supported 1066 FSB processors. They only supported 1066 memory with DDR3