Task Manager. Urk. The figure at the foot of the PF Usage chart is in MB, all other figures are in KB, and the conversion factor is 1024.
PF Usage is a misnomer in Task Manager. But the figure at the foot of the Page File Usage monitor is the Commit Charge, which is actually the sum of RAM in use + Page file being used.
Commit Charge [KB] = curent total memory usage [of both RAM + PF][KB]. It is just the amount of virtual memory the OS has committed to the running programs.
Limit CC = Most of installed RAM + Page File size. Most of RAM? XP always keeps a variable amount of RAM in reserve. If you disable your page file you will see that CC Limit is less than Total Phys MEM [RAM] by about 50KB or so, the reserve. This rises rapidly as you have more processes running, probably because the OS calculates that there is a bigger chance of an emergency memory call occurring.
At bottom of TM you see that PF Usage number repeated as Commit Charge [Total]. The second figure there is Commit Charge Limit, now in MB ...[x 1024 to get KB].
So when you say you have 490992KB of RAM, and your "PF Usage" is 503800KB you can see that you are only using a tiny bit of your Page File. Of course, XP is not going to be using all of your RAM before it switches some allocations over to the page file. How much of your RAM is not being used? This is given by the Available figure. How much page file? Tricky.
And what is shown in the actual Page File Usage monitor and chart? I have no idea.
There is a lot of misinformation on the web about this, I've possibly added to it.