bought new ram, messed up my pc

[hardtarget]

i did a crucial scan on the crucuial website and ordered the recomended upgrade which was this
http://www.crucial.com/uk/store/part...LE=CT12864Z335

my computer wouldnt turn on but a red light came on the front of my pc and the fan was running.
then i put the old memory back in because the new memory wasnt being accepted.
the new ram is pc2700 and the old one was ddr4000 (3).
crucuial scan said i can use both of them at once

but now even after i put in the old memory my computer still doesnt start up.
what could i have done wrong?
has this ram messed up my computer?

Here are my pc specs
aries performa 3000
dual ide udma133 hard disk cable 30cm
k8m800-m2 matx vga 2xDDR 400 lan motherboard
sempron 2800+ 1.6 ghz socket 754 256kb boxed.
256mb pc3200 184pin ddr 400 dimm

this is on the case so its the best i can do,
the computer isnt turning on for me to be able to run some program and get the info

[eth01]

undo the casing.

goto the RAM slots.

double check, that you've clipped the RAM in correctly. (so that its tight)

If all else fails, then you may well have the incorrect RAM for your motherboard. Therefore, you'd need to contact the third-party supplier, which in this case was Crucial.

May seem a little basic, but give it a go!!

[soup dragon]

pc2700 would slow down your original pc3200 memory so i wouldn't recommend you get that anyway.