The reason why you need a fast SDHC memory card is because some features need a fast memory card to work better. If you want to do continouse shooting then you need to save photos fast and if your SDHC card is slow then it can take time.
Right now the fastest type of cards in market are the class10 memory cards. So if you are going to buy one then make sure it says class10. No point of buying a slower can't because it's just couple of dollars cheaper. Also when you are buying memory card make sure to buy from reputed store like Amazon rather than unknown ebay seller. You don't want to end up with fake card.
Transcend High Speed Class 10 UHS Flash Memory Cards are fastest in market and you can get them for normal price. These cards read at 85 Mb/s and write 45 MB/s. Also compatible with all SDHC and SDXC-labeled host devices.
Make sure to buy the correct card for your Camera. Early Canon Bridge camera models like Canon SX10 IS and Canon SX20 IS can use SDHC cards upto 16GB so makes sure to find the type your camera needs. For your Canon SX50 camera you can use upto 32GB. You buy all these memory cards from Amazon for lowest price with free shipping.