The Sonnet Dual G4's are a bad option for a MDD or FW800 G4. The Sonnet CPU's simply lacks L3 cache (and has less L2 cache than Apples CPUs) which makes for instance the dual 1.8ghz slower than a dual 1.42ghz G4 when doing CPU intensive tasks that uses a lot of FPU power (using VST's, RTAS, rendering audio files etc).
I have an "original" Dual 1.25ghz board overclocked to 1.5ghz in my MDD. Have worked great the past two years I've had it, even during the warmest summer days.
Go buy yourself a $50 original Dual 1.25ghz board and overclock it. It's the fastest and cheapest way to maximize the power of your MDD 
I have a FW400 MDD. I started with a dual 1.25, replaced those with 1.42s, and finally I now have one of those rare bird Sonnet dual 1.8s. I can say that without any question, my experience is that the Sonnet is measurably and empirically fastest in all aspects. This is true in both OS9 AND 10.5.8 - L1,2,3 caches and /or single or dual procs being present / used notwithstanding. Running the
same software (which includes StudioVision, Peak, SonicWorx, etc. etc., many VST's, various iterations of Adobe Creative Suite and on and on) the speed bump on each upgrade was immediately apparent - especially with the Sonnet. In OS9 I have no concern about track counts or how many plugins in use. In OSX it almost feels like a G5. With TenFourFox's Quicktime extension, I get full-screen (1920 x 1200) glitch-free 720p video on YouTube all day long. My main bottlenecks now are the 7200rpm drives - my next upgrade will be 2 SSD's. I held my breath when I bought the Sonnet… it was NOT cheap, but it was well worth it. Over the years, I've gone all the way from a Mac Plus to what is the fastest OS9 config there is, as far as I can tell. The Sonnet is the engine that made it happen.
