Yeah, I think the biggest heat sink you can use is defined by the physical space you have available. If you want to use a fan and heat sink, you can use a PC style combo sink/fan, or depending how much room you have (I use A2000 and A1200, never seen inside a 4000) you could use the biggest heatsink that will fit, and try to mount a fan in there drawing air across the CPU horizontally. I've even seen some Amiga accelerators which mount a heat sink on the CPU, and then a fan mounted to the circuit board on the opposite side. I'm not sure if that would be very effective though.
