Want to build a linux system not much bigger than a Mac Mini

I have used Linux as a secondary system since 2004. I am now to the point that I'm fed up with Mac OS X Mt. Lion and Windows 8. I want to move to Linux for 90% or more of my work. I'm already running Xubuntu under VMware's Fusion client on an I5 Mac Mini with 16 Gigs of ram. I recently donated my 2004 Dell Pentium III running the latest Ubuntu to a high school student who is an aspiring CS major. I want to build a compact system probably an I5 based one which will let me live in Xubuntu. I would like recommendations on motherboards, hard drives, cases etc. I don't do gaming but I do store a lot of images. I'm happy with the performance of Linux on a regular hard drive so I don't need to go to a SSD.

I would like the system to be compact and cost less than $500 or $600 not including the monitor. I also write so if I can keep the Linux box at that total cost, I can do hardware comparisons between the Mac Mini, my $499 I5 Lenovo tower, and the Linux box.

You can view my current set up at this link to my View from the Mountain blog. As an interesting aside, I worked at Apple for nearly twenty years and was an executive there my last few years so the fact that I am converting to Linux is a good story of the open source community. It also makes me happy because it gets me off the OS rollercoaster.

I appreciate any help. My son who a network architect with lots of LInux experience can help me with any software issues, but he hasn't built a machine since he was a teenager and his taste runs to gaming and more expensive systems.

Thanks,

David Sobotta
AKA Ocraokewaves @twitter