The latest release is IDS 10 (just released on 25-2-2005), so newer is almost impossible. The release before that was 9.40 by the way. It is very difficult to tell the requirements. For hardware is informix normally not really demanding (1 cpu, 64-128 MB RAM, 0.5-1 GB diskspace). Of course more resources give a better performane.
The software is somewaht more complex to tell, becaiuse the support rather a lot of O.S.'s. Most used are: Sun Solaris, IBM AIX, HP-UX, Linux and Windows. But also other platforms are supported. For more info you can see the IBM website: www.ibm.com/software/data/informix