For 50 users you don't really need anything special, most likely you won't have 50 people trying hitting the server hard at all times.
Is your current hardware rack mounted? If it is get a 2U or 4U server case (so you can fit more than 2 or 3 drive), if not just get a standard tower server case. The last reply said something about 1U or 2U dying like flies compared to "consumer massed produced stuff". I just had to reply to this. What do you think is inside of most of these 19" rack mounted servers? If he builds his own then he can put almost any ATX motherboard he wants into the case. If you get a Dell they have a standard server class Intel motherboard. It may be custom produced for Dell's specs, but it's not like they are using some one off board. The only difference between a tower server/pc and a rackmount server is the case is made for sliding into rails and mounting in a rack. You can put whatever you want in there.
I would recommend getting a Dell. They have special sales all the time where you can get really good deals. If something goes wrong with the server, you can get different grades of support with it. If you get the 4 hour response and you have a raid controller/hard drive/whatever go out, they will have a replacement part to you WITHIN 4 hours. We had a raid controller go bad about 3 weeks ago on a Poweredge 2650. I had a replacement in my hands in 90 minutes. It wasn't even a critical system.
If you must build on your own, make sure to get enterprise class components. If you want to go IDE drives, then get SATA WD Raptors. They are the only "enterprise" class IDE drives that I have heard of. I know mine came with a 5 year warranty, and they are supposedly tested to a higher standard. I would recommend getting a decent hardware raid controller with 3 146gig+ 10K rpm Seagate SCSI drives, plus 2 36gig 10krpm Seagate drives (I just like seagate). Mirror the 2 36gig drives for system files, and then put the 3 146 gig drives into a raid 5 array. If you don't need a ton of system performance you can probably just get 4 scsi drives and raid 5 them instead.
You may also want redundant power supplies (like the last guy said, get two actual power supplies, and make sure they are decent quality. 350W supplies should be enough if you only have a single processor and 4 drives, but more is almost always better. A server class network card is also good(better at handling a large amount of connections than a standard workstation/pc nic). Depending on teh size and useage of your database, you may not need dual processors either. You could always save money by getting a single processor now and adding a second one later if needed when prices go down more.
If you want solid advice on this, please post storage requirements, types of apps that this will be running, and operating system that you will run.
edit: Oh yeah, unless you get an internal tape drive, make sure you have an external scsi interface for whatever drive you get. I prefer LTO/LTO2, but since you probably won't have that much data to backup, get an SDLT drive if you can find one cheap.