The short answer is........... very little.
ATA 5 = UDMA 100
ATA 6 = UDMA 133
Theoretically, 6 has a max burst rate of 133 meg/sec as opposed to a mere 100 meg/sec for ATA 5. However, in reality they behave almost identically - I have a 133 drive on my gigabyte mobo and it's just the same speed as a UDMA 100 one, even though the board is capable of using 133 - not all boards are.
So, to sum up.....even if your laptop is UDMA 133 enabled, you will probably notice no difference between UDMA 5 and 6 drives, so choose your drive using other criteria.