|Did you clear CMOS after installing the new memory sticks ?|
The BIOS may have settings that pertain to the previous memory, such as CAS Latency, RAS to CAS Delay, etc. You have to clear them for the new RAM sticks to work.
I assume you have seated RAM sticks properly.
After installing new RAM sticks, can you go into BIOS ?
Please also tell us how many RAM sticks you have, e.g. 4x1GB or 2x2GB or 1x4GB. Check your M/B manual if the M/B can handle the amount of memory you installed per slot.
You mat also have to adjust RAM voltage manually in BIOS.
Hope the above helps.