upgrading ram, pc will not turn on!!!

[cnmservad]

I have an HP Pavilion a1473w running Vista Home Premium (Upgraded from Windows XP) with 1gb 4200 Ram (2 x 512mb). I just installed 4gb 6400 ram (2 x 2gb). My PC will NOT turn on with either of the two (or both) sticks installed, but if I put the original sticks (2 x 512mb) back in, it works fine. I do not remember hearing a beep or anything when I tried to to turn it on with the new (2 x 2gb) ram installed. HP's website says I can only install up to 2gb 4200 ram, but geek squad told me it didn't matter, it should be fine. Can anyone help me? Please!!! Thank you sooo much in advance. Btw, this computer is the ONLY computer I have...

[cruisin702]

Motherboard specs
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00590356&lc=en&cc=us&dlc=en&product=3204837&lang=en#N915

Single-channel memory architecture
2 x 240-pin DIMM sockets support unbuffered non-ECC 400/533 MHZ DDR2 memory modules
Maximum HP/Compaq approved memory is 2 GB

[patio]

HP's website says I can only install up to 2gb 4200 ram, but geek squad told me it didn't matter, it should be fine.

Typical of the Geek Squad.....take what they tell you with a grain of salt in future.