I have a 64-bit OS! That was in my opening post.

When I had only 2 GB, it showed my RAM as 2048 MB. After adding 2 GB more, it shows 3584 MB rather than the expected 4096 MB.

What is the "it" that shows 3584 MB?

The docs here says it can support 16 GB.

The BIOS, for one. gnome-system-monitor shows the same.

Well, if the BIOS shows 3.5 GB, the problem might be the BIOS. Consider flashing a new BIOS. But first, I'd make sure the problem isn't graphics-card memory, which you haven't assured me it isn't.

I imagine that I do that 512 MB of graphics card memory. But if I can address "64-bits" (actually most only handle only 48 bits at the moment) then this should not reduce the 4 GB.

Right. You were talking to the "Geek Squad" at BestBuy, werent you?

No, why would you say that? I bought the components for the computer at MicroCenter and assembled it myself. I haven't been to a BestBuy for ages.

I can't tell how you intended the statement since your intonation doesn't come through the text. Are you doubting my statement on 64-bit addressing? My understanding is that AMD64 systems set 16 memory bits (actually I think 16 of the middle bits, rather than just the 16 upper bits) to 1, limiting the actual space that can be accessed to 2^(64-16) = 2^48 = 64 terabytes. Let me try to Google up a source,

OK, I have one:
EmbeddedDeveloper.com - x86 - AMD64 (64-bit)
Actually this gives a limit of 48 bits of virtually-addressed memory and only 40 bits (1 terabyte) of physical memory.

Of course these are all better than Vista's 37 bits (128 GB for Vista Business), though I think that's an artificial restriction.