Over the years, I've set up multiple firewalls and fileservers for clients and my own office. What I require is a server platform that is Gentoo Linux friendly with quality components and at an affordable price. Early on, I was a big fan of the Dell PowerEdge 400SC. I remember picking those up (without operating system) for around $200! I stuck with them when they went to the 410SC, 420SC and the 430SC series. The 430SC series is when they changed their look to a sleaker design which is what is available in the new 440SC series.
Dell have been hit and miss when it comes to laptops and workstations, but their switches and SC series servers have been fantastic for me and my clients. If you plan on installing Gentoo on these, you will need to configure SATA in the kernel along with a few other 'gotchas'. If you're interested, I would be more than happy to provide a one of my kernel configs to get you up and running.
These are great servers for the money, and are a perfect solution for a fileserver for any SOHO (Small Office Home Office). Looking to pickup another one, I was blown away to see the entry Dell PowerEdge SC440 at $314.00 at the time of this writing. It's Celeron with 80GB drive, but talk about a deal! I put some other options down below (including a Interal Dual Core option). All of these servers do NOT come with operating systems.