Hello David,
Whether reasonable performance can be achieved at extremely low latencies (e.g. a buffer size of 128) is determined more by the performance and timing all of the drivers in the system, individual hardware components (audio interface, motherboard, graphics chip, etc), and what's installed on the system, than by the amount of CPU power available.
I would regard a buffer size of less than 256 as 'experimental' -- 256 (or 512 at 96 kHz) is more likely to be reliable on most computers. 128 is extremely demanding in terms of hardware/driver/OS timing tolerances, and doubly so if using 96 kHz. I'd recommend sticking to 256 if using 48 kHz, or 512 if using 96 kHz -- each give only about 5 milliseconds of latency.