6.0GB!? That's a lot more than I thought any macbook could support, especially a late 2007 one. I got my Blackbook in July 2007, and it only supports 3GB, although I'm running 2x2GB chips in it because I was stupid and didn't think about it at the time before ordering. It definitely doesn't support 4GB.
Also, I haven't seen a 3GB RAM chip, so seeing as you'll only really have 2 slots for RAM, then in my opinion (although I'm more than willing to be proved wrong) 4GB would be the maximum you COULD go for, if your macbook will support it.
OP, apparently Crucial has an app that can tell you how much RAM your chipset can support ... I haven't used it, think it's on their homepage though.