My computer is rather old. I have only 128MB SDRAM but the motherboard specs says:

• 2 x 184-pin DDR DIMM socket support DDR 333/266 DDR SDRAM
  2 x 168-pin SDRAM DIMM socket support PC133/100 3.3V SDRAM
  Maximum: 2GB

Can I make an upgrade to a DDR? When I do, what would you recommend to make my P4 computer run in its peak capacity? Will it be okay to use an SDRAM with a DDR at the same time? Thanks.

In some cases, you can mix DDR with SDRAM. Best try it out. What will happen is that the speed of RAM you will get will be whatever the lowest specs of what you will mix is. Be careful of different voltages of the RAMs though - if they're different, you'll find an unstable voltage in your system. (That doesn't mean it'll blow up though, but it'll pose problems.)

Yes, you can use DDr for your board. You can actually use two, but you can only have a maximum of 2GB of RAM. You add up whatever DDR you can buy but don't exceed the limit.