High volume use of postGres

I've been charged with investigating postGIS as a replacement for our current Informix runtime (9.3),  which uses the
Spatialblade. Currently we use two Sun Sparc/Ultra 80 boxes with 3 CPUS dedicated to Informix. 

We use our database to calculate the image files we have within a given area (ST_Union, Intersect and the like).

Originally this company tried Oracle, but, as the Firesign Theater put it, "He's no fun, he fell right over!" ...
Informixworks well but charges a hefty amount for each runtime CPU. 

Our databases are not particularly large -- only about 500,000 rows in the larger ones, but we're running more than a
millionhits a day, not evenly spread, natch so at peak we need a lot of capacity. 

Are there documents on performance of postGres on various platforms that I might be able to get some ideas from ? Such
issuesas multiple CPU vs single CPU, Operating System -- we're leaning towards Linux of some flavor but also have some
Sunservers (not as hefty as our current runtime database servers) -- would be of interest. 

Any suggestions from users that have/are really beating up postGIS/postgres would be welcome ! (for instance, cleaning
upa busy database, supporting multiple servers, etc.) 

Thanks,

Greg Williamson
DBA
GlobeXplorer LLC