memory help

[pas]

I have emachines t5048.  Running windows xp 32 bit.  Right now 512 ram.  I  ran a memory test my computer can use PC2 5300 2048 max non ecc  ddr2.  I was able to purchase corsair xms2 ddr2  pc2 6400 800mhz 2x1024 for $5 after rebate.  I put it in and nothing.  Is there any way I can get this memory to work?  Thanks for the help

[patio]

The 6400 may not be backward compatible although it should be...
A few questions?
What MBoard?
Where did the RAM specs come from ?
Did you remove all the old RAM ?
Have you pulled the CMOS battery for 5 minutes to reset the BIOS ?

[pas]

Not sure about motherboard.  How can I figure that out?  I did pull out old memory only two slots. Ram specs from crucial memory scan.  Did not pull out cmos battery.  Need instructions on that. Thanks