When Microsoft introduced Service Pack 2 for Windows 2003 Server, they introduced a bug that prevented any RDP sessions greater than 1600x1200 from displaying in 24 bit color. When Matt Hatley built a fresh Windows 2003 server for an event last August, we could not quite figure out why his full screen session on a dual headed Sun Ray 2FS would only display 8bit color even though we were telling the Sun Ray Windows Connector to connect in 24 bit color. My server worked fine, his did not. Turns out I was not current on my service packs and SP2 was to blame! Sure enough, after applying SP2 to my server, I was limited to 8 bit color as well. Since a customer just ran into this issue, I thought I'd post the fix and some answers.
Bear in mind that this is not just limited to the Sun Ray 2FS, which in a dual head setup runs 3840 x 1200, but this also happens on a multihead setup of any type Sun Rays.
You can apply this hotfix from Microsoft for Windows 2003 Server to get your higher color depth back for your larger RDP sessions.
For the record, you can go up to 4096x 2048 in an RDP session. From a width perspective, that is about 3.2 Sun Ray 270's. Four Sun Ray 270's would be 5120x1024 which is larger than the max resolution in width allowed by RDP. If you wanted a four head setup, you could use four Sun Ray 2's with each screen limited to 1024x768 (4:3 maintained).