I removed the existing sticks and put the new sticks into the same slots. That's one each in DIMM 0 Channel A and DIMM 0 Channel B.
The PC now won't boot, sometimes it won't get past POST, sometimes Windows stops with a bluescreen or a warning about a file version check on boot (it's Vista Home Premium but I don't think it's relevant as POST isn't reliable).
I tried using each stick on it's own in DIMM 0 Channel A and have the same problem.
Is it likely that both sticks are no good? Have I missed something? Am I being stupid?
I would get into the bios, and see if you can adjust the timings manually. These are what I use with my pc3200:
Ras to Cas delay 3 clocks
Ras Precharge 3 clocks
Cycle time (tras) 6 clocks
Bank cycle time (Trc) 10 clocks
Command Rate 2T
I leave the voltage at the default setting. Your board may only have some of these adjustments. Locate the cmos jumper if you need to reset the bios due to a no post situation. If you get to windows, you can try running memtest, a free download. You should run it for at least one hour, with no errors.