There are two parts to the question of running multiple applications on the same server: 1. do you have enough resources? and 2. is there a business advantage to being able to take down a server for one application without impacting the other?
Asone example of the first consideration, Microsoft makes a product called Small Business Server. By design, it runs Exchange v5.5, SQL Server v6.5/7.0, Proxy Server and some other services (fax sharing, etc.) on the same physical server. There are a couple of things you want to do as far as allocating RAM to Exchange and SQL, but I've seen it work nicely with hardware similar to yours and fairly high demands on the Exchange and SQL Servers. As long as CPU and memory utilization remain within rational limits, it works just fine.
And I have to say that 550x2 and 512meg looks mighty good to me - I currently have a Pentium-75 with 128meg of RAM supporting 60 users for file, print services, mainframe access AND Exchange Server. It's wheezing real bad, and CPU utilization often stays near 100% for the better part of the work day but it rarely crashes. Yeah, we'd all like to have 4gig of RAM but it's not always justified.
As far as the second consideration, let's say your SQL Server goes wacko and develops a problem that only a re-boot will solve. Is a short disruption of Exchange services acceptable? If not, then the mission-critical application (whichever it is) needs to be on a separate server (and that server should also not be a PDC or BDC).
To continue from where you are, I would set up some Performance Monitor charts and just watch the system closely until it's stable with the new database application. If resource utilization is then within normal limits, I'd say you have a good situation and can let future decisions be dictated by business need rather than technical theories about what should or ought to be done.
paul