Game server, for ten people, doesn't know much about servers... doesn't sound like a LAN admin to me. I'd guess this is a guy, planning to build a box, that he intends to share with 10 or so buddies, connected to his University's network (I am assuming he is a student).
All that said, and if my assumptions are correct, I'd either find something with a decent RAID controller, or find a RAID controller that fits my budget, and then find a board with the necessary port to plug it into.
IMO, your best bet is to build something that'll support VmWare ESXI (which is free), and run your Ubuntu Server as a virtual host, and perhaps build other virtual servers as needed (to serve games, etc.). You don't have to, but it'll give you some separation between your file server and game server, thus allowing you to toy around with one and not render the other unavailable to the 10 or so people you'd be sharing it with.
As to the hardware, a Quad core CPU is recommended. And as much RAM as is feasible for your budget. 8GB+ would be a good bet.
If this is a personal server, you can save a lot of money by buying SATA drives for your RAID array.
As others have stated though, if you are in fact needing a server for a production (IE - work) environment, definitely buy pre-built with warranty support from the manufacturer.
If it's for personal use though, research what your needs will be, and go from there. You can generally do whatever you need with a low-end quad core, and a heap of RAM. I am running a VmWare ESXI server with an old, old, old Athlon X2-4000+ and 6GB RAM. Have a Windows server connected to my two 2 TB harddrives for a file server, run an Ubuntu server with Apache, etc. etc. for piddling around, and a PfSense firewall for the internet. I'd love to have a quad in that, but I'm pretty sure the wife would kick my arse if I buy any more computer hardware in the next 6 months.