first post, I haven't built a computer in quite sometime but after doing a bit of research and looking at numbers i realized not much has changed. Please let me know what you think, thanks!
antec sonata elite case
antec earthwatts 380w PSU
AMD phenom II x2 550 callisto Black Edition 3.1 processor @ 80 watts
western digital green 1 TB 5400 rpm hard drive
western digital blue 320 7200 rpm hard drive
radeon 5670 512 mb video card
4gb [2x 2gb] mushkin silverline ddr3 1333
ASRock M3A770DE AM3 AMD 770 ATX AMD Motherboard
fedora 12 for now[win7 so expensive!]
So, how "light" is light gaming? The HD5670 got ok but not great benchmarks for gaming. You might consider spending the extra $30 or so and getting the HD5750.
If you can find the Antec Sonata case with the 500W PSU already in it, I think it's a better deal. 500W will also support pretty much any single video card you can throw at it. It has the EarthWatts 500W PSU.
I'd shop around a bit for either one because sometimes you can find around the same price as NewEgg but with free shipping like at Buy.com or Amazon.com.
One other thing to keep in mind. Check the ATI driver support for Linux. I've read it's not so great.
Oh, and if you're getting the Callisto because you think you're going to unlock the other cores, just know that if you want a quad-core then buy a quad-core. There's no guarantees that they will unlock and that if they do they'll be stable. I know I haven't had any such luck. For a bit more you can get a Propus quad-core, like the Athlon II X4 620 which is going to be more than enough for HTPC/light gaming use. I'd say you'd even be fine with the Athlon II X2 3.0Ghz Regor chip. I have a AMD Athlon 5050e with the HD 5670 and my HTPC absolutely flies.
If you do go with the different mobo, be sure to check RAM compatibility. Look at something like the G.Skill Ripjaws series where you can find DDR3-1333 CL7 modules. They'll be quicker. I've had good luck with G.Skill and Kingston modules with Gigabyte motherboards.
Oh, and with about a $20 more you can get the WD Green 1.5TB drive over the WD Green 1TB drive. $20 for half-again as much space. The 1.5TB is the sweet spot right now.