Your comment describes it perfectly (I've reformatted & corrected the one typo):
The Available Memory number of 1.73 GB
comes directly off the System
Information summary page.
Installed Physical Memory (RAM) 3.00GB
Total Physical Memory 2.99 GB
Available Physical Memory 1.73 GB
Total Virtual Memory 6.90 GB
Available Virtual Memory 5.63 GB
Page File Space 4.00 GB
Yes, I was puzzled by the low number.
I was expecting a little under 3.0GB.
The "Available Physical Memory" isn't what your system is able to access, it's what's left after everything running has its memory allocated. Every running program and service eats up some memory, so this seems perfectly normal to me (especially since Vista's memory footprint is fairly large).
Your system recognizes its full 3 GB of RAM. But it has to allocate for all your running software. This value will fluctuate depending on exactly what your machine is running at any given moment, and even what those particular apps are doing.
If you add a 1GB chip, you'll probably see between 256 and 512 meg additional, depending on all of the various hardware resources in the machine, in a 32-bit environment. In a 64-bit environment, you would see the whole GB. However, this will be shown as installed and total physical memory -- not necessarily as "available" physical memory, as again, that value fluctuates depending on what your machine is doing.
In any event, you should note that under normal circumstances in a 32-bit environment, most apps will not be (individually) able to use more than 2GB at a time. However, for the vast majority of end users, this should not be an issue.