postgresql vs oracle vs informix

Hi, I currently have an application with apache, php4 and postgresql, under linux. My final platform will be a big unix machine (say SUN) with Apache and php4. I have to decide which database I will have, as good performance is required. I read somewhere that postgresql is as fast as oracle, but even if it is, I don't think it has the possibility of spliting the database between several machines, as oracle does. On the other hand, I have wasted a lot of time trying to install oracle on my linux machine and I don't want to go through the same nightmare in Solaris. Also, as they don't offer a robust and good installation script for Linux, and they don't offer support for developers at all (they never answered my questions), I am seriously considering using Informix instead of oracle. Please send comments, and/or let me know a place with some comparation benchmarks. Thanks in advance. Pedro.