just performed some tests with different reserved mem sizes (stock kernel ZSJP8) for your ref

default settings:
CONFIG_ANDROID_PMEM_MEMSIZE_PMEM=16384
CONFIG_ANDROID_PMEM_MEMSIZE_PMEM_GPU1=10240
CONFIG_ANDROID_PMEM_MEMSIZE_PMEM_ADSP=1500
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC0=14336
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC1=1024
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC2=12288
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC0=36864
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC1=36864
CONFIG_VIDEO_SAMSUNG_MEMSIZE_JPEG=5012
CONFIG_VIDEO_SAMSUNG_MEMSIZE_TEXSTREAM=10240



1/. minimum (almost) reserved mem with movie playback possible (no lag in youtube), 356mb memory, no special lags observed, opened 4 tabs in browser with lots of imgs and swfs



CONFIG_ANDROID_PMEM_MEMSIZE_PMEM=8192
CONFIG_ANDROID_PMEM_MEMSIZE_PMEM_GPU1=4096
CONFIG_ANDROID_PMEM_MEMSIZE_PMEM_ADSP=1024
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC0=6144
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC1=1024
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC2=6144
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC0=29696**##
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC1=29696**##
CONFIG_VIDEO_SAMSUNG_MEMSIZE_JPEG=3072
CONFIG_VIDEO_SAMSUNG_MEMSIZE_TEXSTREAM=2048

** min value in order to play movies = 29mb, 28mb wont play
## can be set to a lower value without movie playback, setting these with too low values caused problems, below 20mb should be ok


2/. nearly double all default values, 219mb available, didnt feel faster




as to min mem usage, u'll need to (in brief):
1/. remove unneccessary apps
2/. remove startup/network chg broadcast listeners from the app - a lot of apps/services are launched at startup up or network state chg, remove them so they wont start automatically, eg gtalk, sync/feed, vold service, installd, etc
3/. kill unneccessary services & processes - i didnt use any killer since i rooted my phone, i use script to kill them since killers consume more mem
4/. i have read the source and found that only -Xmx (max mem) is included but no -Xms (initial mem), perhaps we can do something here (not done yet)


test2: available memory - 257.68mb




reference for 2.1
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC0=10240
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC1=1024
CONFIG_VIDEO_SAMSUNG_MEMSIZE_FIMC2=10240
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC0=24576
CONFIG_VIDEO_SAMSUNG_MEMSIZE_MFC1=24576
CONFIG_VIDEO_SAMSUNG_MEMSIZE_TEXSTREAM=8192


more info