I just added another gig of memory to my HP Pavilion a1340n. It came with 1 gig so I bought a PNY 1 gig Pc Memory DDR2 PC2-5300.
CPUZ sees Both RAM sticks but when I right Click "My Computer" only 1 gig is said to be in use
Alright, the memory is working fine as CPU-Z is recognizing the memory. Reset the CMOS and update the BIOS and then install the memory. It should work. The latest BIOS update can be found in the HP support site <--(Click here)
FYI, sometimes the actual available memory will not be the exact amount of memory you have installed. What happens is that some of the addressable memory (regardless of how much you have physically installed) is reserved for use by page files or by some of the devices that you are using, such as a graphics card, PCI card, integrated network connections, etc., so it's unavailable for use as normal main memory. Hope that helps.