I'm running 161/161 with my KT3Ultra-ARU and an XP2100+ (unlocked, 11.5 multiplier). The limitation in my system appears to be the processor, though I actually managed to get as high as 179/179. Problem with that was that the system was actually slower overall than the current 164/164, since I had to lower memory timings (CAS2.5, 5, 3, etc. vs. CAS 2 , 4, 2, etc.). From my tests, running a lower FSB but being able to turn those memory settings down (lower numbers, faster response) makes a bigger improvement. Also, for some strange reason, even if I lower the multiplier to the point that the processor is underclocked with the higher fsb setting, the system gets unstable. The RAM itself handles 166 and aggressive memory timings fine (as proven by using the default asyncronous 133/166 with aggressive timings just fine- indicating that it's the increase on the processor's side that causes the issues).
Ironically enough, the processor has just once completed a 3DMark2k1SE run at 1904Mhz, for my highest result ever (the GF4 4600 was unoverclocked), but the system hung not long after recording the results at Madonion's site. I was planning to play with overclocking the card until that happened (trying to break the 12k barrier, lol. Oh well, I'm content with 11-11.5k).