May i suggest that you join the RCX400 group on Yahoo?
http://tech.groups.yahoo.com/group/RCX400/This was -just- discussed, along with diagrams of the wiring.
But yes, you can use a USB to serial adapter (i recommend the Keyspan USA-19HS as the most trouble-free) and the Meade 507 cable (a 505 will work, but it has a 4-pin plug that will sit loosely in the RCX rs232 serial socket)
have fun
--dick