When it comes to H.264 VA-API video decoding on Sandy Bridge, the Core i3 2100 is still plenty capable and works fine assuming you are using the latest Linux 2.6.38 kernel DRM and the updated libva library.
In most tests the Core i3 2100 performance is only down by 7~8% compared to the Core i5 2500K boasting a similar graphics engine, but with some OpenGL applications this puts the integrated Intel graphics under Linux closer to the brink of not being playable at higher resolutions or with ramped up visual settings. The Core i3 2100 still though does provide modest integrated graphics abilities and is more than suitable for use on a composited desktop with Compiz, KWin, or other compositing window managers.
The H.264 VA-API decode support for Sandy Bridge is also great (it's not just using shaders like some proposed Gallium3D work) and the encoding support is expected to land sometime soon. With that said, the Core i3 2100 could make a very modest processor for an HTPC / media PC.
Intel's Core i3 2100 Sandy Bridge is currently retailing for $130 USD or less at NewEgg and Amazon. Remember though for a pleasant "out of the box" experience you will need to wait for Ubuntu 11.04, Fedora 15, etc, or be updating the key packages (Linux kernel, Mesa, DDX, libdrm, libva) manually.