By default, an all-64-bit PC will still have the standard big holes in its memory from three to four gigabytes. This is the lowest-hassle way to deal with the problem - just install more than 4Gb of memory, and live with the fact that your 8Gb PC with a 768Mb graphics card only actually has seven-point-not-much gigabytes of visible RAM.
One advantage of this is that you can still boot a 32-bit OS, if you want to. Another is that this vanilla configuration is most likely to actually work. Cleverer memory configurations aren't necessarily properly supported by hardware and OSes yet.
If you don't care about these factors, though, there are two ways to get the lost memory back.
Some 64-bit motherboards these days give you an option for "memory hole remapping". That moves the fourth-gigabyte MMIO memory holes higher into the 64-bit address space, probably way above the maximum RAM you can physically install.
Many other 64-bit boards, though, are even smarter, and can leave the memory holes where they are and remap (at least some of) the physical RAM out from under the holes and up past 4Gb. This process is often entertainingly referred to as "memory hoisting", and it used to be the preserve of server motherboards. It's been showing up in more and more desktop mobos, though. And on some of them, the memory-hoisting BIOS setting even works, and doesn't horribly crash the system as soon as something tries to use the remapped RAM.
You may only be able to "hoist" the last 512Mb of the 4Gb address space, but that's better than nothing. If it works.
