Imagine this: (slightly wrong, but will help you get the picture)
The amount of pipelines is like a "multiplier" for the core speed (it doesn't affect the memory speed, the "multiplier" for memory speed is rated in bits, ie, 256bit or 128bit) so, if you have a two cards with a 500mhz core clock, and one has 20 pixelpipelines and the other 24, then this is the difference:
20pp:
20x500=10,000
24pp:
24x500=12,000