You should use this URL to get the user to record type selection screen
This one is for Account and you can change it to case (500 is prefix for case) by ent=Case.
If this needs to be used for any custom objects using ent=mycustomobject__c wont work.
To make it work for custom objects too you will have to use the 15 character id of that custom object. To find it, go to:
Select your object and check the URL in your browser(for example: https://cs87.salesforce.com/01I8E0000008XjE?setupid=CustomObjects)
This is the id that we need(01I8E0000008XjE)
Then, just replace the ent=Account with ent=01I8E0000008XjE
Hope it helps.