Luckily, my iMac is not encumbered with such odd capacities, as it will only take up to 2Gb (ok, so that's not really better at all :) and will perform better with matched pairs of SODIMMs, so 2x1Gb is the ideal configuration for it.
I didn't know about the performance optimization with matched SODIMM pairs until this morning. I just bought 1x1Gb SODIMM to put in my 512Mb-equipped Jan '06 iMac (1x512Mb, I think/hope--yes, they shipped at least some without the optimal configuration for performance). I think I'll be ok with 1.5Gb of memory at not entirely optimized performance rather than 512Mb and massive swapping. :)