Just b/c the light is on, dosent mean that the psu is giving enough power to the system. Mabey the psu died but there is still a tiny amnt of power going thru it for the light. This auctually happened to me, the psu would boot the pc for 1/2 sec, then it would shutoff cuz there was not enough power. Try replacing the psu and see if it works.
To use the HDD, u will need an IDE cable if you have your CDROM and existing HDD on the same ide chain. If they are not, u can add it as a slave to either chain, make sure to set the jumper. It will be seen as a hdd in windows with all the same files n everything, just make sure taht your orig hdd is set to boot from first.
And as long the RAM is the same type (most likley 184 pin DIMM DDR SDRAM) they will both work, however, all sticks will run as fast as the slowest stick. Example; I have a stick of DDR 400 RAM and i add a stick of DDR 266, both will run at 266 speed.
How to identify your RAM.
Good luck witjh your problem.
P.S. Make an informative title next time