Ya know, you can pick up DDR500 (if the board supports it) the only problem is last I checked, it was ridiculously expensive.
One thing you have to know about Netburst, timings and latency do not matter. Its shear brute force architecture, the more of it and the faster is the better.
I wish I still had my old DDR match-ups, i'm sure I could pair you a couple 1GB's... alas, those days are over
P.S. Question, is your core a Northwood or Prescott ?