Say your Rat is around $100 i would get an SnG to go with it. Come to think of it, if I had $500 thats probably what I would use it for. An RC-3 and an SnG. Assuming this is US dollars of course, $500 AUD buys me about $0.10 USD.
EDIT: While i was typing macgregor22 posted his suggestion, and it is good to. Maybe even a bit better. I would go with his.