This will probably be migrated to serverfault.com soon, but here's my experience:
Some DB's don't recover from a crash well if their temp files are destroyed upon reboot. I don't know if this is the case with Informix. Also, the RAM is often better put to use as DB buffers (at least, that's my experience with MySQL). That way, the DB can use it in the most efficient manner possible. If a solid state drive (SSD) large enough to hold your temp files is in your budget, that could be a good investment.
If you're dedicating a hard disk, that implies that you have more than one available. Consider using RAID 0 to combine two or more hard disks rather than use a separate hard disk. This gives you nearly twice the read/write performance in my experience (based on benchmarking done with MySQL, with my application transaction mix).
Stripping "unnecessary" files won't really increase performance, stopping unnecessary processes will. Assuming you're not low on disk space, files that aren't being used don't hinder your performance much (they can contribute to disk fragmentation, but files that were installed with the OS and never altered probably aren't fragmented unless the disk was highly fragmented to begin with).
Speaking of fragmentation, ensure the disk holding your data is regularly defragmented. That can have a big impact on your performance if there are many inserts and deletes.