Question what should i set VideoMemorySizeMb= i just got a new video card ( I had a ATIRadeon HD 4800 512megs before ) now i have a Nvidia Geforce GTX 75 TI (2gb) and i have 4gb system ram.

[MEMORY]
ExpandSystemMemoryX64=true
ReduceSystemMemoryUsage=true
DisableDriverMemoryManager=false
DisablePreloadToVRAM=false
EnableUnsafeMemoryHacks=false
ReservedMemorySizeMb=512
VideoMemorySizeMb=2560 // VRAM + RAM - 2048 ?
EnableCompression=true

I use,

VideoMemorySizeMB=

AutodetectVideoMemorySize=True

I leave nothing as a value, but Your Video Card V-Ram is the value You use if You want a value in VideoMemorySizeMB.

From what I've read, you're supposed to set it to however much VRAM your card(s) have, which in your case would be 2gb (2048).

So it shoud be this then ?

ReservedMemorySizeMb=2048
VideoMemorySizeMb=2048 // VRAM + RAM - 2048

ReservedMemorySizeMb=512
VideoMemorySizeMb=2048

Probably more like this. I'm not 100% sure what the 'ReservedMemorySize' is (probably system RAM allocation?), but most profiles come with it set around 256 or 512, so it probably doesn't need to be as high has 2048 - it may even make your performance worse, but again I'm not certain.

Were is AutodetectVideoMemorySize= its not in the enblocal.ini did you add it in ?

i have 4 gb of system RAM not 512

Sure, but then you don't really want Skyrim to able to hog all that RAM up, since it doesn't need it.

Edit: In http://enbdev.com/enbseries/forum/viewtopic.php?f=2&t=2241&start=150#p34525 Boris himself says that he finds 512 is best for him, so going with that seems sensible.