> following in my /var/log/messages:
> 
> Apr  2 19:08:11 pc-grunza-p /netbsd: WARNING: mclpool limit reached;
> increase NMBCLUSTERS
> 
> My PC has 1GB of RAM.  What would be a good value for NMBCLUSTERS?
> 

you could easyli overshoot it much to maybe 4096 (256 or 512 is default)

NMBCLUSTERS are not preallocated statically, it only limits maximum value. 
with 4096 it would be 4096*2048=8MB maximum.