1 GB is not very much in Splunk terms -- just about any modern server hardware should be able to handle it.
There are a lot of potential variables, so it's difficult to throw out specific specs. That said, the docs suggest a base spec of 64-bit cpu, 8 cores, 8 GB RAM, and 4 reasonably fast disks in RAID10. With only 1 GB of data, you may be able to go lower.
As for OS choice, if you want to index Windows data, especially Active Directory, you are better off running Splunk on Windows. Otherwise Linux is your best choice, assuming that you (or your sysadmin) already is comfortable with Linux administration.
Take a look here:
http://www.splunk.com/base/Documentation/4.1.5/Installation/CapacityplanningforalargerSplunkdeployme...
and to a lesser extent here:
http://blogs.splunk.com/2009/10/27/add-a-server-or-two
