将sql server 2008 R2 express版升级到标准版

I am using sql server 2008 R2 express edition,it only supports 10 Gb DB space. how can i get know how much space is used and how much free space i have balance,i have tried sp_spaceused, but i didn't get clear answer. i think i have 2 DB's. and if i want to upgrade 2008 R2 express edition to 2008 R2 standard edition what is the procedure?, can i upgrade directly or need to take backup and uninstall express edition and then need to install standard edition?. is that anything i need to pay (for license) to upgrade to standard edition?I am using sql server 2008 R2 express edition,i