"Is 1333MHz much better, i.e. a noticable difference to 1066 when it comes to RAM?"
Very slight.. That 25% speed increase would feel more like 5% or 10% (If it's even noticable at all) On the other hand a low Latency can also increase the speed of the memory and if your current memory has a higher latency and you buy all new memory running at 1333 with low latency it would likely feel more noticable.
"Any advice on whether the 1333 is worth the extra outlay?"
I really don't think the 1333MHz speed will seem fast enough to justify the extra cost. I would just stick with 1066 and upgrade the AMOUNT of memory to 8GB. Your are almost ALWAYS better off going with MORE memory over faster memory.
"...adding 4GB 1333Mhz to the current 4GB 1066 is not advisable."
That is correct, DO NOT BUY MEMORY THAT IS NOT THE EXACT SAME.