I wish to add more memory to my HP Compaq de5800, it currently has 2 x Elpida 1gbs installed with a further 2 slots free, could you please advise me on what size, type & memory I can install? mhz etc
What OS do you have on it?
For a 32bit system you don't need more than 4 GB total (it doesn't even fully use that). For a 64bit you can keep adding.
Memory (DIMMs) PC2-6400 (DDR2, 800 MHz) DIMMs Non-ECC
HP 2-GB PC2-6400 (DDR2 800 MHz) DIMM
HP 1-GB PC2-6400 (DDR2 800 MHz) DIMM
HP 512-MB PC2-6400 (DDR2 800 MHz) DIMM
I still don't know what OS you're running on your system.
Assuming you have a 64bit, then you can go for the max memory supported, either in 2X4GB or 4X2GB sticks ( 2 slots or 4 slots occupied).
The best way to ensure speed and stability is to have all sticks similarly rated; otherwise, the system will only run as fast as the slowest component, and the slower RAM sticks will dictate that.
In your last configuration you would have 6GB total RAM, which is also OK for a 64bit OS. If I were you I would just upgrade the whole thing and go for maximum allowed by the mobo. Memory sticks are not that expensive, esp. if you don't shoot for the fancy ones that might not even be supported by the mobo, and will run at the limited rating anyways. Just get some PC2-6400, because getting a higher rated stick will not be utilized by the mobo, the RAM will be limited to 800MHz by the FSB.