I: Rom paging method has been reverted to the pre XXX releases. Seems a bit smoother in a few games. 128MB should still be supported (UNTESTED).
This fixed some slowdowns like Mario Kart 2 player using 1964 and Rice 5.1.0 (probably other also). There was a noticeable slowdown between Surreal 1.0 / FDB and XXX that doing this resolved.
If by doing this - problems arise for 128MB users - both paging methods can be implemented and set based on total system memory.
IV: Increased Dynarec and Paging mem from 16 to 20 (max in the ini) for 64MB users. For 128MB users, Dynarec has been increased to 32MB and Paging to 64MB
(although the paging value should be not be used with systems with 128MB RAM because the entire rom is supposed to be loaded in memory - tinker with it

).
I'll read through the B5.1 thread and see what I can find regarding this issue. I don't recall any changes being made with 128mb sources, but I'll have a look. It could very well be that something was broken in B5.1, but lets hope for a work around.
Looked through the B5.1 thread; looks like you were the one reporting it all before. Its a shame this was over looked, but it probably would have to be reverted to fix the crashing.