So I got a fairly decent couple of fans: an Akasa 80mm fan for the front mobo side (blows on the disks) and an Enermax 92mm for the back. I also put some Coolermaster RAM heatsinks on one side of my 512MB DIMM.
The result? I can safely take my 1733MHz XP2100+ up to 1898MHz (equivalent to a hypothetical Athlon XP2325+). Or 1% more than before!
This bodes fairly well, as it means I can put an XP2600+ in, and run it as an XP2873+ (nearly XP2900+) which is pretty sweet.
Motherboard temperatures are down to 30C (from 43C) and the CPU is cruising at 46C (down from 52C). Pretty sweet. I've a 12cm Akasa fan to put somewhere, but will need to ensure I don't screw the airflow up. I reckon with branded memory (CAS2) I should be able to squeeze even more out.
I did get the machine to run for a while at 1911MHz (XP2342+) but then it went a bit haywire :-( Oh well!