1080p hardware acc. sandy bridge IGP?

Do XBMC support 1080p hardware accelerated playback (h264) in both windows and linux using Intel IGP (Intel Core i5 2400 Sandy Bridge)?

Or do you need to pair it with GT430 to ensure hardware playback?

Windows yes, Linux no (with the default XBMC Live).

With Linux you ALWAYS want an Nvidia card. A GT430 has all the bells and whistles (aka 3D support), but even a GT210 will play every 1080p file you can find (I promise, I have tested more than anyone).

With that said, an i5 can tear through anything on any OS by itself.