Which...

[The_Techie]

It is a known fact (and being a Computer Engineer does help). However, the computer won't say that it is running at whatever FSB when you use slower RAM.

I'm sure it does

Setting a FSB speed is related directly to the speed grade of memory a system must use. The memory bus connects the northbridge and RAM, just as the front side bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Increasing the front-side bus to 170 MHz in most cases also means running the memory at 170 MHz.

In newer systems, it is possible to see memory ratios of "4:5" and the like. The memory will run 5/4 times as fast as the FSB in this situation, meaning a 133 MHz bus can run with the memory at 166 MHz. This is often referred to as an 'asynchronous' system. It is important to realize that due to differences in CPU and system architecture, overall system performance can vary in unexpected ways with different FSB-to-memory ratios.

In complex image, audio, video, gaming, and scientific applications where the data set is large, FSB speed becomes a major performance issue. A slow FSB will cause the CPU to spend significant amounts of time waiting for data to arrive from system memory.

Source.

EDIT: Note that I am merely correcting your statement that slower RAM lowers the FSB. It plainly doesn't. Let us take my computer as an example:

It uses a 1,066MHz FSB, which translates into a 266MHz bus speed, which is multiplied by 9 to achieve a 2.4GHz internal clock speed, regardless of the RAM that is used. The slower the RAM, the longer the CPU will wait for instructions/data/whatever you want to call it from the RAM, however the FSB will still be 1,066MHz. If it was true that "667MHz RAM slows your FSB to 667", then the bus speed would be lowered to approximately 167MHz which would result in a 1.5GHz internal CPU speed. Your CPU will have more idle time, though To summarize: The speed of the RAM does affect system performance (by how much seems to be debatable) but it does not lower your FSB.

 

[The_Techie]

If you want to run your RAM in a 1:1 RAM:FSB (or FSB:RAM, I forget which ) ratio (which seems to be desirable, since there is no waiting states between the FSB and RAM) instead of using any other of the asynchronous ratios, DDR2-667 is too "fast" for an 800MHz FSB You will need to use:
DDR-400 for the 1:1 ratio on an 800MHz FSB
DDR2-533 for the 1:1 ratio on a 1,066MHz FSB
DDR2-667 for the 1:1 ratio on a 1,333MHz FSB
DDR2-800 for the 1:1 ratio on a 1,600MHz FSB

 

[Glordit]

If you want to run your RAM in a 1:1 RAM:FSB ratio (which seems to be desirable, since there is no waiting states between the FSB and RAM) instead of using any other of the asynchronous ratios, DDR2-667 is too "fast" for an 800MHz FSB You will need to use:
DDR-400 for the 1:1 ratio on an 800MHz FSB
DDR2-533 for the 1:1 ratio on a 1,066MHz FSB
DDR2-667 for the 1:1 ratio on a 1,333MHz FSB
DDR2-800 for the 1:1 ratio on a 1,600MHz FSB

My MSI G31M2 mobo hit's 1,333 MHz FSB with DDR2 800 RAM [acording to the manual ] although that would be in theory, I should actualy try it before confirming this.

EDIT:
• Intel® G31 Chipset
- Supports FSB 800/1066/1333MHz.
- Supports dual channel, DDR2 667/800.
mmmmm

 

[The_Techie]

My MSI G31M2 mobo hit's 1,333 Mhz FSB with DDR2 800 RAM [acording to the manual ] although that would be in theory, I should actualy try it before confirming this.

It will be able to run DDR2-800, because then it will use a different ratio (and not slow down to an 800MHz FSB ) I'm just saying that for the 1:1 ratio purposes what speed the RAM should run at I'm not saying that you need a 1,600MHz FSB to be able to run DDR2-800, but you do need it to be able to run it with a 1:1 ratio.

 

[Glordit]

It will be able to run DDR2-800 regardless, because then it will use a different ratio (and not slow down to an 800MHz FSB ) I'm just saying that for the 1:1 ratio purposes what speed the RAM should run at

Ah! okay, thanks for that. but seems as if max ratio I get out of it is 1:1.25 or 1:2 - could be mistaken

AAAH!? I think it has to do with the fact that you need a CPU that supports a FSB faster than 800 the E2160 max Bus speed is 800

 

[The_Techie]

Ah! okay, thanks for that. but seems as if max ratio I get out of it is 1:1.25 or 1:2 - could be mistaken

AAAH!? I think it has to do with the fact that you need a CPU that supports a FSB faster than 800 the E2160 max FSB is 800 if I am not mistaken

Yeah, it means that the motherboard supports the E2160's native FSB of 800MHz all the way up to a 1,333MHz FSB (unless I misinterpreted your question/statement)

 

[Glordit]

Yeah, it means that the motherboard supports the E2160's native FSB of 800MHz all the way up to a 1,333MHz FSB (unless I misinterpreted your question/statement)

Nope! hit the nail on the head!
The motherboard support's from Celerons's/Core Duo/Core 2 Duo Quad core all the way up to the new Intel 45Nm CPU's hence the 800FSB - 1333FSB support