First, you don't need NT to run
Tomcat, it runs fine on Windows 98.
Second, if you do choose Windows NT (or Windows 2000), you really should get more memory than 64MB. While NT will run in 64MB, as soon as you start up any significant applications it will overstep its memory and begin swapping to disk.
Java does not like swapping, it really slows down the system whenever garbage collection happens. 128MB should be OK depending on what else you will want to run. Windows 98 can often uses less memory, if you are really limited by the laptop manufacturer to 64MB, then consider running '98 (or even '95) instead of NT.
A 600MHz CPU will give you perfectly adequate performance. Spend your money on memory.