You should be looking at something like the Pentium e5300, e6300 or Core 2 Duo e7xxx.
DDR ram should work. DDR speed isn't really that important although you can use DDR 2 if you want.
Your IDE hard Drive is safe but yoy can upgrade to SATA later on.
The mobo format is ATX. Count the slots on your current board. If it is more than 4 slots it is ATX.