RAM Swap Trouble

[NitroJ223]

Due to some incompetence I can't figure out the problem. First off I am running an ECS k7s5a motherboard. around 1.1ghz and The RAM currently in it I believe is PC2100 128mb DDRAM. The problem was when I swapped in 2 PC2700 dual channel 512mb RAM sticks. Everything was working awesome then they decided to cause some problems. i.e. the computer would not boot after the RAM check and boot devices. I was told that PC27 and PC21 are backwards compatible and I wasn't sure if there was a particular setting that had to be changed in BIOS for everything to work properly. I understand how touchy RAM is, but I was curious if someone could help a dim bulb brighten thanks.