ultima @ 6 heatpipes > noctua @ 4 pipes
only exception would be if noctua was 8mm pipes
I found a couple reviews (older version of the Noctua) and their performance was about the same. Maybe the newer Noctua is slightly better? But unless you can go to a larger size cooler I'd think your best bet is to keep chuggin with the Ultima's.
Number of heatpipes is NOT everything... placement of the heatpipes on the CPU and even distribution of the heatpipes and joints with fins has a LOT to do with it. One of the reasons why the orochi with its full 8 heatpipes (not 4 heatpipes in a U shape which are then "counted" as 8), gets beat by 3 heat pipe designs like the Vendetta (let alone the Vendetta 2 with its direct attached heatpipes).