Re-Seat the RAM..

Your mother board may not like the ram.. it may not run with anything over PC2700 or the new ram maybe less than it minimum rating of PCXXXX.. see that is the information your not telling us.. What is the rating of the old memory..and the new.. single and double also has an effect on the mobo on how it addresses the ram slot.. (had lots of probs with the PC133/100/66 mems)
ALSO.. have you tried starting the system with the original ram in place.. you may have disturbed something else while fiddling around in the case..


Also