I guess I'm in the minority, but I say 4 GB at 1066. Why? You want your FSBs matching in an ideal case. Assuming you get Q6600 or something similar, it has an FSB of 1066. To optimize performance, the two values on your processor and Memory should match.
Now of course, if they don't match it's not a HUGE deal, but know that for all memory transactions, your memory will be slightly lagging your Processor.
At the same time, the argument about expanding I think also supports going 4 GB at 1066. 800 is a dying breed. 1066 is already out there and 1333 is on the horizon (as is DDR3) so I think matching things up now is the best solution. Also with DDR3 this entire argument may be moot (but my estimates are it'll be awhile for that...oh and you'd have to get a new processor/mb for that too...so forget DDR3)
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Also from personal experience, I only have 2 GB of 800 RAM and it works fine. Then again, when I edit it's usually just a few layers and nothing fancy, but yeah the two vids I've worked on with this machine weren't a problem. Then again, I'm OLD SCHOOL -_-;;
...and wish I had more RAM
