Running all 6Gb together will work but it will run with half the throughput of using only 4Gb.
Reason being, your DDR2 RAM runs in Dual Channel Mode
, it accesses data from pairs of DIMMs simultaneously, somewhat like a RAID setup.
When you run an odd number of DIMMs, your memory controller will default to single channel mode, accessing each stick individually.
Unless you run an application that benefits from more than 4Gb of RAM, you will see no boost in performance from running 6gb over 4Gb.
Quite to the contrary, your performance will actually drop by doing so.
Personally, I would either run a single pair for 4Gb of RAM or locate another 2Gb stick that runs at the same timings and voltage for a total of 8Gb.