B1 bounds is 44M at the moment.

For B2, it's generally sensible to use the default value when performing stage1 with gmp-ecm as well.
When you have few memory, there are 3 options:

1. use smaller B2 bounds
Curves will be completed faster and need less memory, but more are needed for a certain digit level.
For 49e9, I need upto 340MB.

2. use the parameter "-k <x>" where <x> is a number
By default, x = 4. It's basically the number of fractions the b2 "space" gets divided in.
Increasing this value saves memory, but curves take a bit longer.
Quadrupling the value halves the memory consumption.

3. use the parameter "-treefile <x>" where <x> is a name
Now, some tables are written to disk instead of stored in memory. This considerably reduces memory requirements, but of course lessens performance, but not as much as one might expect.