Go to Siliconinvestor.com's stock screener and group stocks by sector. Unlike a blanket list of sector constituents, the screener will enable you to refine your sector search by specifying whatever price and volume constraints you want.
I checked one basket (GSO) and saw Informix listed as a component of this software index. IFMX was swallowed up by IBM (I think) at least a year ago. I would not trust this source as being accurate and/or updated.
The other thing to check is are you looking at volume from the AMEX website or your trading software? A huge % of the volume is done on Island which won't be listed on the AMEX website.