Upgrade canon 300d or buy new lens

Hi, I currently use canon 300d with the 18-55mm (old, non IS) kit lens and a 50mm f1.8. I mainly take portraits, family and holiday pics. Would now like to upgrade my kit. Am considering canon t4i and tamron 17-50 f2.8 . Due to budget constraint I can only buy one of these this year. Any views on which of these investments will give a bigger improvement ... Also if I go for the T4i, should I get the 18-55mm IS kit lens... Thanks .