|The problem with making two changes at once is now you don't know which of them is causing the problem.|
If the HDD you installed is PATA/IDE, make sure you set the jumpers correctly. And if the new drive is sharing a cable with another drive, you should double check the jumper on the existing drive as well. One must be master & the other must be slave...or both set to cable select.
As for the memory, try taking out 3 sticks & see if the system will boot with just one.