ok just researched....4 DIMMS at stock clocks of under 100MHz is fine, over 100MHz most can't get 4 DIMMS to work, regardless of registered or not (the BX6r2 actually has an elaborate TI memory buffer setup, but get unstable over 100MHz).
still I got in on Crucials 10% + $10 off deal and picked up a 256MB stick to replace one of my 128MB's which will go to my father since he is trying to run Win98SE with 64MB....640MB for me and 192MB for him

.
Later I will add a third stick for 768MB and he will get 340 or 256MB (I heard the Abit BH6r1.1 may not be able to handle 3 DIMMS)
Is 512MB the sweet spot still for XP Pro?