for 40 - 50 users it is not going to need to be particularly powerful.
i dont have a lot of experience with servers, but the main server for all students at my school has only a dual core pentium d, and 4gb RAM, we never have any problems really, but thats not to say it couldnt be better.
that hp should be ok, but it is better to seek professional help, it would easily be powerful enough, and you can easily add new drives should they be needed.
What kind of applications? Pre-built may be the way to go only because of support. You can do fairly well with an entry-level server which is often not much more than a desktop-grade PC with better storage options for a multi-user environment.
Coming from a single core CPU I think you could easily run everything you need with one quad core. Not only will you get a speed boost with getting modern CPU architecture but you'll still have 3 more cores than you currently have. No need to spend more on a dual CPU server.
I would say if you don't want to think about getting a new server for a long while get 16GB of RAM and 2TB of hard disk space.
Go with 8GB of RAM and 1TB of disk space if you want though. You may have to reevaluate the server sooner though. Not sure on the specifics of the files/applications you are using, but things are bulkier than they once were.