I need to add RAM but when I opened the tower to put a new one in, I discovered both slots are being used. Can I purchase an external hard drive to give me extra RAM? Will it speed up my PC?

Hi Bspra
Welcome to G2G
An external hard disk will not speed up your PC/
How much ram is in your pc now ? Right click on my computer icon then click on properties this will show you what ram is installed. How much ram did you buy ?
Cheers
Peterm

I have 512 but won't removing one of them and replacing it cause me to lose stuff I have saved? I returned the RAM I bought.

No - The RAM is what the computer uses while it's running. The hard drive is what everything is stored on permanently - the RAM gets cleared every time you reboot the computer.

Well, darn and drat, now I have to reorder the RAM. Thank you for the info.

One more question, please, before I order the RAM. These are the specs from my computer per Tiger Direct:
HP Pavilion 540n
Product Type Personal computer Processor Type Pentium 4
Data Bus Speed 400 MHz Chipset SiS650
Native OS Microsoft Windows XP Home Edition Memory Technology DDR SDRAM
Max Supported Memory 1 GB Memory Speed 266 MHz
Memory Specification PC2100 Upgrade Rule Max 512 MB module
Memory Slot Type DIMM 184-pin Memory Slots Qty 2
Free Memory Slots 1

DO I UNDERSTAND CORRECTLY THAT THE MAX MEMORY SUPPORTED IS 1 GIG AND I CAN ADD A 512 TO THE 256 I HAVE?

Hi bspra and welcome to Geeks to Go!

According to Crucial.com your computer can hold a maximum of 2048 MB (2 GB) of RAM.

Now, you only have two slots from what I can see...

In any case, if you have one slot currently in use with a stick of 256 MB RAM, then you should be able to add that second 512 MB stick in there. That will give you 768 MB which will work very nicely. You'll notice a big difference.

As Facedown says your computer will support 2 gig of ram
click on me
In your 1st post you say you are using both slots. So this means you will have to remove 1 stick of ram and put in a larger stick of either 512 or 1gig - If you can afford a 1gig stick then that would be better. If you can afford 2 gig that would be great. Let your budget decide. I would try to strech to 1 gig at leasst.
Cheers
Peterm