Dimesion 8400 Adding RAM memory

Hello,

I am wondering whether I should add more RAM to my Dell Dimension 8400.  It had an HDD crashing issue after I installed a new HDD, but that has been solved by installing the Intel Storage Matrix driver.  The problem that remains, versus when I purchased it six years ago, is that Windows XP Professional loads slowly and most applications (e.g. Internet, Outlook, Excel) load and function slowly too.

I looked at the Windows Task Manager, specifically the 'Performance' tab last night after doing some online research.  My 'Total Commit Charge (K)' was greater than my 'Total Physical Memory (K)' which, as I read, indicated a need for more RAM.  My 'Peak Commit Charge (K)' was also consistently greater than my 'Limit Peak Charge (K)' which was also an indication of a need for more RAM and/or to increase the virtual memory.

As a first step, I increased the virtual memory.  The maximum virtual memory had been set at 1.5x physical memory by the factory.  The initial virtual memory was considerably lower, so I changed it to 1.5x physical memory as well.  

I had read that over-reliance on virtual memory can slow down the system.  What I have found though, is that startup of Windows XP Professional is considerably faster.  The launching and responsiveness of applications (e.g. Internet, Outlook, Excel), however, remains hit and miss, but mostly slow.

In terms of performance, the 'Total Commit Charge (K)' continues to be higher than 'Total Physical Memory (K),' I'm assuming, because I have not yet added any RAM.  But the 'Peak Commit Charge (K)' is now considerably lower than the 'Limit Commit Charge (K).'

Should I buy more RAM?  I'm trying to understand the rationale behind a purchase, so I'd appreciate confirmation that my perspective is correct (or not).  My memory speed, based on what is installed, is 533 MHz.  Therefore, if I buy more RAM at 533 MHz, the computer's 'speed' won't actually change.  The speed will always be 533 MHz.  What will change is the capacity of the physical memory.  At the moment, I have 512 MB (2 x 256 MB) of physical capacity.  In order for my system to function, even if there aren't a lot of programs running, it is currently relying on that 512 MB of physical memory PLUS a lot of virtual memory (i.e. PF Usage averages around 800 MB).  If I buy 2 GB of RAM at 533 MHz (2 x 1 GB) and install into DIMMs 3 and 4, I will have 2.5 GB of RAM at 533 MHz.  This greater physical capacity will materially cut down on the reliance on virtual memory, which will making the loading and operation of any number of applications, few or many, seem 'faster.'  Is this correct?

If it is correct, here's the link for what I intend to purchase: http://www.valueram.com/datasheets/KVR533D2N4K2_2G.pdf

Specifications: