Apple's website says it's shared with main memory, and that 348 MB is the minimum.
Intel's website says the HD 3000 will only use 1.7 GB of RAM maximum (Windows 7 though, no mention of Mac OSX).
source: http://www.intel.com/support/graphics/sb/CS-029090.htm#max
Since we can't make the chip use a set minimum or maximum on a mac, I don't think it's worth swapping out the 4GB sticks for 6GB or 8GB sticks, unless you do heavy multitasking.
Intel's website says the HD 3000 will only use 1.7 GB of RAM maximum (Windows 7 though, no mention of Mac OSX).
source: http://www.intel.com/support/graphics/sb/CS-029090.htm#max
Since we can't make the chip use a set minimum or maximum on a mac, I don't think it's worth swapping out the 4GB sticks for 6GB or 8GB sticks, unless you do heavy multitasking.