'Optimal' is as fast as you can get it to run at as low a voltage as it takes to do so. I buy a lot of G.Skill F2-8500CL5D because it will always run well above rated specs, at well under rated voltage:
The comparison to an AC hoist motor is invalid. We're not talking about overloading an extension cord here... A stick of DDR2-800 pulls about a watt and a half; the 2.1 most fast RAM specs is a 17% overvolt - hardly appreciable. The higher speed causes it to dissipate more power as well - so you're probably looking at two watts (and notice that nearly all fast RAM is 'wrapped' in some sort of heatsink) - one third of what your tiny little night-light bulb consumes... Lower voltages
do not 'stress' circuitry (higher voltages do - the term to google is 'electromigration' - it actually [gradually] eats the transistor junctions), the parts simply cannot reach the higher rated speed at JEDEC 1.8V...
You can always use faster RAM with any modern CPU/MOBO setup - you're just likely to have to set it up manually in the BIOS to take advantage of it. All DDR2 ram is actually DDR2/800; they 'speed-bin' it, i.e., test and select the sticks that will work at either lower (faster) latencies, or higher (faster) speeds, or both, and sell it at a premium as 2/1066, 2/1200, and so on. JEDEC spec'd RAM has a little EEPROM chip in it that stores the set-up information/tables for running it at 800 at various FSB (Front System Bus) speeds - has the preferred memory multiplier and timing info - this is called an SPD (Serial Presence Detect) just to confuse us; faster, higher rated sticks may (but don't necessarily) contain another set of tables (called an EPP - this one makes sense - Extended Performance Profile) that will tell the BIOS what multiplier/latncies to use at its higher rated speed - BUT - not all BIOS are created equal: some will read this EPP automatically, and set the RAM at the higher speed; some will require intervention (on a lot of Gigabytes, it's "Load Optimized Defaults" [but, to keep it more confusing - not all BIOS with the "Load Optimized Defaults" fuction actually use it to set the EPP]), and some just plainly don't know the EPP exists (if it does) and you have to set the higher speed manually!
You have control over the basic system clock (I'm going to cal it B_CLK), once you start manually timing the MOBO through the BIOS. B_CLK times four is your FSB (once again, Front System Bus); B_CLK times your memory multiplier is your DRAM rate; B_CLK times your CPU's multiplier is your CPU frequency.
Examples: if you set your system clock to 333, you will need a 2.4 memory multiplier (333 x 2.4 = 799.blahblahblah) to run your RAM at 800, and if the CPU multiplier is, say, 8.5, you will get a CPU clock of 2.83GHz; at that same B_CLK you would need a memory multiplier of 3.2 (3.2 x 333 = 1065.6) to take advantage of 1066 RAM. Now, lots of CPUs that are rated at a 1333 nominal FSB will run a lot faster, sometimes with a little more 'oomph' from a voltage increase; for example, I run a Q9550 that is rated at 1333 FSB (333 B_CLK) times an eight point five multiplier, for a 2.83GHz speed. It will comfortably run with the B_CLK well over 450 - and here's where faster RAM comes in. The smallest RAM multiplier available from a MCH (Memory Control Hub - or 'NorthBridge') is 2.0, but, with a 2.0 multiplier, that means at a 450 clock, your RAM will need to run at 900 (again, 450 B_CLK x 2 = 900), which most 800 RAM just won't do! This is referred to as a 'RAM limited bus', meaning the CPU can't run a B_CLK any higher than (roughly) half the RAM's available speed - and thus, the need for faster RAM. Mind you, this only applies if you both can, and intend to, run your FSB above 1600 (once again, a B_CLK of 400+ times 4 gives you a 1600+ FSB)...
To further complicate matters, people often misunderstand the actual quantitative speed improvements inherent in faster ram... Here's the mistake: 1066 is 33% higher than 800 ([1066-800]/800 = 266/800 = .33), so 1066 RAM must be a third faster than 800, right? Not so! You have to figure in latencies. Most 800 will run at 4-4-4-12, while most 1066 is rated at 5-5-5-15, or, even worse, 5-5-5-18. Here's how to appraise the situation in reality: at 800 MHz, a RAM bus cycle is 1.25 nSec long (1000/800); at 1066 (1000/1066), it is roughly .938 nSec long - so, with an 800 stick at a 4 average latency, a RAM bus transaction takes 1.25 (cycle time) times 4 (latency), or 5nSec, while at 1066 it is .938 (cycle time) times 5 (latency), for a transaction time of (roughly) 4.7nSec - so you see, by going to nominally 33% faster RAM, you actually gain three tenths of a nSec per transaction - .3 (transaction gain) over 5(transaction total) = .06, for a real-world improvement of 6%. The main function of fast RAM is to give you more 'headroom' to OC your CPU!
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