The 5120C HP Pavillon uses PC-100 memory.
There are some PC-100 systems that have problems with 256MB modules built using (32x8) chip configuration. In other words.. 256MB SDRAM single side.
Check your current memory, if it is single side then most likely you can use another 256MB built with (32x8).
If not, your best bet is to buy a 256MB module using (16x8) double-sided memory configuration.
(16x8) configuration for PC-100 is more widely accepted in PC systems than (32x8).
PC-133 memory will work as PC-100, so you could buy PC-133 too.
Also... you'll be better off buying CL3 memory, as opposed to CL2 memory for SDRAM. However, you can use <A HREF="http://www.cpuid.org/download/cpu-z-128.zip" target="_new">CPU-Z</A> to check what latency your PC is using. The value is listed on the memory tab as CAS# Latency.
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