If you're on Windows XP x86 (32-bit) or Vista x86 (32-bit), the PC will only be able to use 3gb total and will show up as 2.75gb in the control panel. If your PC is still under warranty, you can request a x64 OS disc from Dell and you can use up to 8gb of ram. The 2x512mb sticks you already have will need to be removed and replaced with a 2gb module in slot #1 and a 2gb module in slot #3 in order for your ram to be used to its full potential. Adding memory will increase the speed/responsiveness of your PC to a point. It will also depend on how many programs are starting up when you log onto Windows and how powerful your CPU is.

Some additional comments.

1.  Dell's are notorious for being picky with memory brands and there is no pattern, one brand will work in one PC and may not work in another same model.   Most of the "forum experts" recommend Crucial memory   www.crucial.com  for non-Dell 3rd party memory as it is one brand that is guaranteed to work in Dell's.  Wherever you buy the memory, make sure they have a "liberal" return policy in case it doesn't work in your Dell.

My Dell with XP Media Center and 4GB of RAM shows 3.24 GB of RAM out of the 4GB.  On my home built PC with Vista 32 bit it shows 3.321 GB of RAM in the Task Manager (but Vista with SP1 will show how much actual memory is installed even tho it can't use (address) the full 4GB).  

thanks for the responses.   I do not have a great knowledge of computers....so, I'll ask "how do you know if the computer is using all the RAM? Where do you find this info. I now have 2.5 G of RAM. I was wrong about 2X 500 mb .....I had 4X 250 mb . So I removed 2 of the 250s for 2 of the 1Gs. I guess I see the difference....hard to tell. Thanks.

The Operating System manages memory and assigns it or uses it as needed.  With 2.4GB you are under the "less than 4GB ceiling" for 32 bit OS and need not be concerned.  The Task Manager will show you the memory and also what applications are using whatever (however don't be concerned if you see something in the area of 50 to 60 more processes as that is normal).  CTRL/ALT/DEL will bring up the Task Manager.

Thanks for the respose Fireberd. I have trouble understanding what the task manager is telling me so I am not really sure where the RAM usage information is located. Anyhow, I do feel like I have more speed.....but with 2.4 G of RAM, now I want to have 4Gs even though it seems 2.4 is very adequate. Maybe I'll order 2 more modules in the future. What do you think?   Thanks again.    Ron

What more memory does, in general, is (1) allows more programs to run (be open) at one time and (2) allows for more operating system "cache" (temporary) storage rather than having to use hard disk space for the cache which is slower (although I doubt that you would ever notice any difference). 

Unless you usually or plan on having the need for a lot of programs open at one time or some high end games, basically all the extra memory will do is lighten your wallet.