How is that RAM actually configured? 3x2GB? 2x2GB + 2x1GB? 4GB + 2GB? If it is 3x2GB, adding a 4th DIMM (assuming it has the same structure as the other three) would allow full dual-channel bandwidth which would provide a little extra performance (usually less than 5% unless using IGPs) on top of possible benefits from having 2GB more RAM.
ignore hte post above me.... you should have 3x2gb, as the original i7 used Tri-Channel ram.
it's not worth changing, as noted by captainTom.
if you tried to add another 3x2gb setup to get to 12gb, most motherboards will just give you errors as the memory controller fails to handle 6 seperate sticks.
that's been going on for years in the first gen i7