My PC currently only has 1GB of RAM, and I am currently using 86% of that. With 2GB of RAM, would I be using 43% right now?
Yes, your math is correct, to some extent.
However, note that modern operating systems use excess memory for caching. This article does a good job of explaining how OS memory management works (written for Windows Vista, but generally relevant for all operating systems); basically, it tries to use up your free (idle) memory for its cache, and when needed by another program, that memory is released again.
So the OS might appear to be taking up more memory, but it is only for caching purposes and can be freed at any time.
My company recently upgraded their XP laptops from 1 GB to 2 GB and my machine is currently using 1 GB of RAM, so XP probably uses more, too.
It's not like your memory is only RAM. There is also the pagefile, which is basically used for the same data. Just think of RAM as the faster part of your system memory. If the OS notices there is more RAM available, it will use more of it to store data that would normally reside in the pagefile. But if needed, the memory can be released quickly again, so that's nothing to worry about.
No, you'd probably be using less than 86% of that. It's like the way a family lives in a house. If you have 10% of the house empty and you move into a house twice the size, you eventually wind up with the new house about 90% full as well. Managing space is an active process, and more space means you can keep more stuff. There's no point in having more space just to keep more of it empty.
Free memory is just a cushion, a necessary evil in case the system needs space faster than it can free it. Otherwise, the system does its best to use all available memory. That tends to result in the least disk I/O and the most responsive system.
Having recently upgraded my RAM, I know that XP will use more for buffering and caching purposes when it has more to mess with. With 768mb RAM simple services in Windows like spoolsv.exe (printer related) would use about 1 to 1.5mb of RAM. With an upgrade to 2.5gb of RAM spoolsv.exe is now hogging 4mb, as are many other services that previously used around 1mb.
It is important to understand that memory usage in Windows is highly dynamic. Memory usage is dependent on system workload and how much memory is available. When more memory is available the system will enlarge the system cache and process working sets. When memory is short then working sets will be trimmed back. Physical memory usage is controlled almost entirely by the system with applications having only limited influence.