Yes, the N900 only has 256MB... so would doubling it to 512MB help much? Absolutely, but would quadrupling it to 1GB give you significantly more benefit than you would get with 512MB? Highly doubtful.
1GB would be nice, but it comes with drawbacks, and when the battery capacity is already likely to be marginal it doesn't seem sensible to take more out of the battery for what may turn out to be negligible benefit. Remember you'll always have bags of swap available for when you need it, which won't be nearly as often with 512MB than it was with 256MB.