Okay....just remember that heat is killer.....dont know if you ever saw a high end gaming pc they have fans everywhere in the housing and the heatsink is something that you can stand on......and some of them are watercooled all this just to play a game, lol....might as well just get a Psp.
Ha, yes I've seen those gaming machines, they sound a vacuum cleaner with those fans.
I have an old Sun rackmount server. When it is running you wouldn't be able to tell whether a vacuum cleaner was working or not. It is a bit like sitting beside a helicopter.
It does run cool though.
But they need to be in parallel in order to keep the current per cell down to safe limits. A 6 cell battery pack will normally be 10.8V so three cells in series and another stick of 3 in parallel with that. Assuming 2400mAh cells that gives you 4.8Ah
For a laptop consuming 65W (Which is pretty high these days) at 10.8V it is pulling a hair over 6A from the battery. Each cell is seeing 3A of that which is still on the high side for 18650's though within safe limits. With a 9 cell pack, you will still have the same voltage, 10.8, but you now have 7200mAh capacity and 2A from each cell which ought to mean that each cell lives longer as they aren't being worked so hard. The maximum power draw from most laptops nowadays is only seen when charging a depleted battery.
The parts cost for the cells in bulk is probably of the order of $3-4 per cell. Add a few dollars for the protection circuit and plastic casing, a 9 cell pack shouldn't cost much more than about $90 retail. But they all cost quite a bit more than that. And for older laptops they can be hard to find.