In broad terms, you are correct.
Regarding memory usage, it's a little more complex than just using the extra 1GB of RAM. There's physical memory, devices that use memory mapped I/O which aren't RAM (Bios/CMOS, Communications, PCI Buss/devices, and installed devices all consume addresses), and then the addresses needed to communicate to/from the physical resource. There is a hard limit of address space of 4GB 'worth', against the installed RAM and Devices which all use the same address pool. This is the "32bit limit" you are referring to when you say the OS will see another GB of RAM.
There's some other software~based fact of life to add to the mix: 32bit Windows (XP and Vista) allocates up to 2GB for the system and uses the other 2GB for applications. Whereas while 64 bit (again, both XP and Vista)still allocates 2GB for itself, but doesn't impose such a limit on apps since there is plenty of address space.
What this means is that in 32 bit all of the apps you run are still limited to a 2GB 'playground' of address space while the OS keeps the other 2 to itself. 2GB for application is a lot of space, but newer games - especially strategy stuff like CoH and graphics/video apps - can and sometimes do use that much all to themselves, and possibly more if it were available.
Now - Going back to the hardware aspect, devices do consume addresses, and video cards are the hungriest. A 64 bit os has the space available to allow you to go nuts with devices, and still have plenty of room to fully address RAM. So later if you wanted to upgrade to a Crossfire setup, then if you were using 64 now then you would still have access to the full 4GB of RAM. If you were using 32 bit and wanted to make the same upgrade, then what you pick up in video you could/would lose in RAM. So, all else being equal, then it'd be best to make the jump sooner and once, rather than skipping now and perhaps having to do a reformat and clean OS install later after you've accumulated more on your system.
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