Ram: 4 gig of 800 Ghz or 8 gig of 667 Ghz ?

[speerchucker]

HELP! I need help from someone that REALLY knows pc's.

I'm mostly a Mac guy, but am putting together a new pc-based live rig. I will be using the micro-ATX format so I can keep the rack case @ 16" deep, but this limits my choice of mobos.

The best microATX mobo currently available seems to be the Intel DQ965GF. This will support a 2.40 Ghz Core Duo quad (the latest versions will) but the ram choice is limited.: I can use EITHER 4 gig of 800 Mhz or 8 gig of 667 Mhz.

My question is two-fold:

1: With a given CPU, how do you determine "how fast is a waste?" The 2.40 quad's FSB is 1066 Mhz. Is it always advantageous to match ram speed to FSB speed, or are there other limiting factors?

2: How the heck can you figure out how much ram you need, before building & running the system? This system will be running a host (Sonar or Ableton) plus a whole bunch of VSTi's. No recording whatsoever.

I assume that maximum ram will lower the amount of time the VSTi's such as Ivory and Kontakt will have to stream from the disk, and that should lessen cpu overhead, yes? BUT HOW MUCH? Arghhhh .....
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The tail that wags the dog here is latency. I want to keep my HW buffer at 128K or lower. 6 ms makes my flesh crawl. I assume I need to minimize cpu overhead as much as possible. -But while 8 gig @ 667 Mhz might ease the disk-streaming, will it kick me in the butt as far as all other processing? Am I ultimately better off with 4 gig of 800 Mhz ram?

If size didn't matter (it does, no matter what SHE says) I'd get an Antec Take3 case with an Asus P5K3 Deluxe mobo, pop in a quad extreme and get the fastest ram out there, but how ya' gonna put a 20.5" case in an SKB rack? Feh.

Any thoughts / suggestion / info will be most welcome.