Media PC Requirements

As long as you have a graphics card capable of decoding H.264, H.263, VC-1 and MPEG2 via DxVA then you won't need much CPU power at all to power through even Bluray spec. H.264 or VC-1. Your G43 should do that job fine, as will any HD3000series or above and GeForce8000series will do that.

For my HTPC build I used a XFX 9300 mATX motherboard as it's based on nForce730i and has integrated Geforce9300 for full DxVA acceleration. For HTPC build going with AMD has many advantages over Intel including, but not limited to cheaper cost, more energy efficient, greater variety of integrated graphic solution supporting H.263 DxVA, etc.
The reason I went with Intel was because I had a spare E6300 sitting around doing nothing and I use my HTPC for small server tasks as well which needed a little more grunt than average HTPC needed.

E5200 is more than enough for HTPC. Just be wary the DxVA implantation by Intel on G4x series chipset isn't as flexible as what ATi/nVidia provides. Bluray playback with DxVA acc. should be fine with PowerDVD 8(or the latest 9), but rips (especially torrented) might give you trouble if more extreme encoding parameters were used.

Head to www.silentpcreview.com for more specific info on HTPC builds.

My bad, didn't read the Ubuntu bit. Does Linux even provide an alternative to DxVA? Because DxVA is a DirectX=M$ thing afterall.
WINE? (Just stabbing in the dark here)

I'm more familiar with Windows and used Server 2008 x64 for my build (with a couple of hacks to make it HTPC friendly).