These are two examples of pump tops:http://ncix.com/products/?sku=34148&...ia%20Co.%20Ltd
They increase the performance of the pump and allow to use any fittings that you want.
The stock top of the MCP350/355 only accepts 3/8" tubing but a custom one allows to use any G1/4 threaded fittings, whether it is for 1/4" or 1/2" tubing.
The stock top of the MCP655 is already using 1/2" ID barbs so it's not an obligation to get a custom top if you're planning on using 1/2" ID tubing but it would improve performance a little and allow you to use compression fittings if you prefer those.
I ordered about 75% of my watercooling equipment from NCIX to prevent having to pay customs fees from US stores.
I strongly recommend pricematching everything with DirectCanada.com since they're cheaper but their customer service is not up to par with NCIX.