Why do you need to upgrade? Have you not been applying patches, for example. Or maybe your company stopped support from Oracle.
That is a 10+ year old box. This link shows available OSes as of 2008:
Sun Fire X2200 M2 Server Overview
None of the links on that page work because they point to sun.com and oracle pretty much short circuits all sun.com links to a single oracle front page.. So, you may need an oracle logon - which will require having paid support on your box to access lots of features.