Canon EOS 600D memory card ?

The memory card provided with camera was a Class IV 4Gb Sandisk Ultra 15Mb/s SDHC card. I want to buy another card with larger memory..So is it better to go for a usual 8 Gb Sandisk SDHC card or 8Gb Sandisk Ultra 15Mb/s SDHC card. As the later one is coslty. Iam just an entry level photography lover. Same way i dont know how bigger card will this device support? Please do help me answers for my 2 questions?