Of the ones you list, RTF and TXT are the two formats that you can read on the 505. But you should also look for ebooks in LRF. That's the native format for the Sony Reader. LRF works much better than anything else. You can find LRF format ebooks at Munsey's
as well as here in our ebook library. There are many more places to find them, but here's two to get you started.
P.S. Before you leave, try to find a retailer that has a 505 to sell you. Demand has been high lately and as a result supply is iffy.