There are too many variables involved to answer your question directly. So, I will answer with an example.
I run an Iwill KK266 with an Athlon 1000 at 1500MHZ. This is a killer mobo and I got lucky with a great Athlon 1GIG that wanted to ROCK!. Anyway, the system runs stable at 150FSB*10, with the RAM at Cas2 and fastest setings. I have also run it at 158FSB*9.5 for 1501MHZ. I use ToniCom PC-166 RAM.
I had stability problems at 158FSB, I am pretty sure this was due to the RAM, as I have more than adequate northbridge cooling and this motherboard is known for going much higher. There are very few motherboards that can do 150MHZ stable, much less 160+, but I feel that the Iwill is no doublt one of them.
There are SO many variables involved, but I will attempt to address your questions as well as I can:
1: A decent KT133A board can easily be run at 150+ safely.
2: At even these levels of FSB, the RAM timings are more important than ultimate FSB. For Instance, My system has better memory benchmarks at 150FSB with settings of Cas2 and the fastest bios RAM settings than at 158FSB at Cas3 with the slowest settings (all my ToniCom could muster).
Conclusion: If you already have some good PC-133 or better RAM around, grab and Iwill or Asus mobo and party on, if you are starting over or fresh, just go and do the DDR thing.
You can, with a lot of tweaking, DDR levels of performance with a KT133A board and good RAM, but if possible, why not just do the DDR?
I love my Iwill KT133A system, but if I were starting from scratch, I'd probably go DDR.
OCnMan