Best Memory for Stock Q6600 Based System

[tintax]

Apologies for making my first post in a while (I've been ill) a question but my lack of knowledge has been bugging me all day!

I'm starting to think about building a new system for my brother. I'm pretty sure I'll be using a stock clocked Q6600 CPU (I've been known to undervolt systems but never to overclock them). I'm now trying to work out what memory would be most appropriate. If I quickly run through my thinking, perhaps someone would be kind enough to point out anything I've got wrong and take a stab at identifying some RAM.

The Q6600 has an FSB of 266MHz (quadpumped to 1066MHz). In an ideal world, where both the FSB and RAM moved data continuously at full bandwidth, I would run the memory synchronously (a FSB:RAM ratio of 1:1). For DDR memory this would be 533MHz since the effective clock speed is twice the actual clock speed.

However the delay between requesting data from memory and it being retrieved ultimately affects the actual throughput. These operations are what is referred to by the latency timings (such as CAS) specified for RAM modules. To a degree this latency can be offset by increasing the memory speed (and thus bandwidth) but is subject to diminishing returns. Unfortunately, running the memory asynchronously (i.e. at a different speed to the FSB) also introduces an additional delay whilst the CPU and RAM resynchronise.

For example, running memory at 667MHz (a FSB:RAM ratio of 4:5) would likely be slower than running at 533MHz because the extra bandwidth doesn't compensate for the latencies. We need to reach speeds of 800MHz (2:3) to see performance gains.

With the current prices there seems no point buying anything slower than DDR2-800. What I can't decide is if it is worth spending the extra for low latency RAM or for DDR-1066. Which of the following is likely to offer the most value for money?

4GB (2x2GB) Corsair XMS2 DDR2-800 (5-5-5-18)
TWIN2X4096-6400C5
LN19302 £58

4GB (2x2GB) Corsair XMS2 DHX DDR2-800 (4-4-4-12)
TWIN2X4096-6400C4DHX
LN21658 £70

4GB (2x2GB) Corsair Dominator DHX DDR2-1066 (5-5-5-15)
TWIN2X4096-8500C5DF
LN22020 £85

I should probably point out my brother can multi-task like a daemon, doesn't view gaming as the primary purpose of the box, and will probably insist on running some version of Windows (sigh). I'll be hoping the box lasts for 3+ years with minor upgrades.

[staffsMike]

Core 2's aren't greatly effected by latencies anyway and going for PC8500 with a stock q6600 seems a little overkill.

I wouldn't bother with corsair DHX modules as they are huge, bigger than they look in the pictures.

Option 1 will give ample performance imo.

[chuckskull]

4GB (2x2GB) Corsair XMS2 DDR2-800 (5-5-5-18)
TWIN2X4096-6400C5
LN19302 £58

You'll be more than happy with those, just make sure you have 64bit OS or you can only use 3.5gig(ish)

[tintax]

Core 2's aren't greatly effected by latencies anyway

Is there a simple explanation for why this is? I'd guess it would have something to do with an efficient cache system, such that data is requested from RAM in fewer and bigger chunks rather than frequent and smaller chunks but I really am guessing here.

[chrestomanci]

Is there a simple explanation for why this is? I'd guess it would have something to do with an efficient cache system, such that data is requested from RAM in fewer and bigger chunks rather than frequent and smaller chunks but I really am guessing here.

That is the simple explanation. Core 2's have loads of cache and a very good branch predictor so cache misses (and fetches from system RAM) are rare. Part of the reason for this is to compensate for the relatively slow memory subsystem in Core 2 systems, where all memory fetches have to go via the north bridge.

In comparison, AMD CPUs have gotten away with much less L2 Cache for years because they have an on-board memory controller, so the penalty for fetching from system RAM is lower, but this means that they are more sensitive to slow RAM because they use it more.

[Noxvayl]

I agree with what the others have said.

For overclocking though the dominator's are beasts(I mean they overclock very well so if your brother likes doing that he might enjoy them more). Other than overclocking your best choice are the first options

[tintax]

Thank-you everyone for the simple and unambiguous advice.

I'll admit I'd been dreading the "well you've got this, this, and this wrong - you really should consider hidden options 4, 5, and 6" replies. It's a pleasant surprise to find I've muddled together a basically sound understanding.