I have tried two class 4 microSDHC cards as second swap devices on N900
a) Polaroid (PNY) 8 GB Class 4
b) Kingston 4 GB Class4

The Kingston one provided much better performance with a 768MB partition on it as a swap device.

I am now planning to get a Transcend Class 6 microSHDC card (this card has rave reviews about it's high write throughput) and use it as additional swap space.

IMHO 256MB RAM just doesn't cut it for the N900 loaded with applications many of which run in the background.