Well, I can't imagine SQL Server handling your requirements, and I find it unlikely a mission critical system like this will use MySQL or Postgres (primarily because Enterprises like to pay a lot for that 'security' of support), so I'd shoot for Sybase if its not Oracle?
The reason I ask is because I am a pretty highly skilled Oracle guy, and some of the numbers I have seen around financial systems just seem so fast and big volume transactions per second. Maybe I could get Oracle to do them, but then the zero downtime scares me somewhat as until very recently, keeping Oracle up all the time, even through application upgrades is fairly tricky!