Welcome Misferatu.
As gameboy says above it's as simple as correctly formating your SD and running that script at startup (which it will do automatically when you save it to /etc/event.d ) .
I'd say 384mb is a good bet, as I personally haven't ever seen the swap spill over that, the default 786mb swap the n900 originally came with is massive overkill - and remember that original swap will still be available if you do manage to fill the swap you created on the SD card.