1) Does a 512 MB GPU mean that 512 MB worth of addresses are lost from main RAM?
No - Just whatever portion happens to be memory mapped. And no, there isn't an exact calculation available. It depends on the exact card, driver configuration, driver version, etc etc etc... You haven't provided any information on the exact components you are looking at. Therefore it isn't reasonable to ask for an exact number.
2) Does doing SLI/Crossfire almost necessitate a 64-bit OS, because so much RAM will not be usable?
Depends on configuration. You haven't provided anough information for someone to even make a half a$$'d guess. What cards? How much RAM? What Mobo? Onboard or aftermarket sound? If so, how much? The answer you want/need will be completely different depending on things like whether your SLI config is a pair of 9800's, or a pair of 295's.
Nor have you provided any information as to your intended usage for the system - Do you run one game? Do you run a bunch of other stuff in the background?? Do you encode video while gaming? In other words,
"How much memory is actually *required* to run the things you want to run?
All of those scenarios have different memory and addressing requirements. Without knowing what components, there's no way to even begin. And without knowing what apps, how many concurrent apps - and therefore the expected maximum amount of memory usage - there is no way of judging whether or not how much address space -
(that we have no way of guessing will or won't be available because we don't know the hardware) - that could potentially remain available is sufficient for your needs.
3) How much address space do DVD drives and HDD's take up?
Almost none. A few KB for the address for the device itself.
The bottom line is if you want 4GB of memory or more, then right there you have smacked straight into the 32 bit address limitation. Period. End discussion.
If you're willing to spend big money on a server edition of windows, then make the effort to install and configure the necessary codecs, audio, video, multimedia software/drivers - Writing ones which are Large_Address_Aware as necessary - in order to take advantage of PAE, then good for you. Then you can stay 32 bit and use all your RAM.
My personal advise is to use new components, a 64 bit OS, and don't worry about it. Because then address space is a non-issue.
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