I first installed it on an external HD, updated to 10.5.5 after that I slimmed it as much as I can (well I can slim it even a bit more deleting the kext files not in use in /system/library/extension), I have installed Parallels,CrossOver Games, Flip4mac, Perian, FFScroll, X11 and Soundflower (to workarround the mute problem with sound driver).
I used CarbonCopyClonner to copy my slimmed Mac OS X to my 8 Gb SSD
and used Type 11 Installer V6 to make the disk bootable.
I moved the Applications to my 16Gb SDHC Card. I left only "System Prefences" and "Activity Monitor" on he SSD
disk. I have created symbolic links to point things that seem to be a little weight to my SDHC Card, like CrossOver Bottles Folder inside Library, and Mercury Folder. With Symbolic links you can leave even more space in the main hd partition.
I have a GUID parted HD with a 6.83 Gb Main partition and I have around 1.5 Gb free right now, and plenty of space on my SDHC card.
Mac OS X on Main partition: around 5 Gb used
Mac OS X Applications (Not needed but recommended) on SDHC: arround 1 Gb
Of course it fit in the 8 Gb disk, but it is better have as much free space as possible on the main partition.