nforce2 chipsets do get a small benefit from running two sticks of ram (dual channel), but its not nearly as much as they'd like to claim... something along the lines of +5% memory bandwidth. Not a big concern really.

Cheapest is to use the stock heatsink that comes with the retail 2600+. It'll work fine if your case is not stuffy and you are not upping the clock speed or cpu voltage. You could always upgrade the cooling later.

The memory cas rating is a factor of timing, or rather delay between certain memory operations. Running at cas 2.5 is pretty standard, running at cas 2 is even better. Make sure to get PC3200 so if you ever do want to overclock, you can take that CPU up to 3200+ speed by upping the FSB to close to 200.