I have Asus P5N-D MoBo with Q9550, and 2x2 gb RAMM KVR800D2N5/2G.
And I have 2x2gb RAMM, that is not mounted, but is a litle difference...The other ones are KVR800D2N6/2G.
The diference is N5-N6.
Would they work ok toghetter?
I don't see a list for timings on these but the CAS latency is different and some Intel boards don't like the low profile DIMMs. I've got Intel ASUS and ASRock boards that so far haven't accepted any low profile memory. My AMD builds with ASUS and ASRock support it well.
The only thing I could see is that it doesn't boot or isn't stable. Usually the BIOS will adjust the timings and frequencies if everything is set to AUTO for the memory in the BIOS. Also in the BIOS there is usually an option on ASUS boards that allows you to mix and match memory by down clocking speed and latencies to match the slowest stick in the system.
If it doesn't boot up, just remove them and position them in different sockets, and try again. You will not hurt anything if you handle the DIMMs properly. If it does boot up and appears to work, run a memory diagnostic like memtest86+ to see if there are any errors. Sometimes mixing and matching memory will cause errors if they BIOS doesn't handle them well. Errors can also be bad sticks. But you want to check for errors if it is working and boots. That way your system won't randomly crash in the future... you will know now if you have any issues.
I browsed for your BIOS layout, and it was a bit confusing, but it seems that in BIOS, usually entered by hitting DEL when the PC starts on ASUS motherboards, you should navigate to Advanced > Chipset > Memory Timing Setting
The first four settings are what you want to change according to your ram specification (which you can read using CPU-Z if you can not find them on the memory modules or the package in which they came).