iPod touch Which IOS versions can I upgrade to

Hello; Current date is 16.05.2012
I have IPOD touch with IOS 4.1 I
Which IOS versions can I upgrade to without shsh files? How can I find currently certificated IOS list?

I want to upgrade to 4.3.3 I downloaded ISPW file and entered UDF mode. With Itunes I got error.

here is why I want to upgrade:
PeelApp Version 1.8.0
+ Fix: GPRS now fully functional on iOS 4.3.3, 4.3.4, 4.3.5, 5.0
+ IMEI Registration requirement removed
+ Various stability, interface and other fixes
+ App renamed to “PeelApp”

Is there anyone who think a solution for me.
Regards