Don't look for chipset - that's not the reason for the low performance in encoding. Look for "priority" that's the reason the X2 had low performance in encoding... also read the stuff about the number of threads or processes - and how much slower the PDEE was at the other tasks while its encoding was 'hogging' cpu time even though it was set at low priority.
Mike.
PS: I don't remember where (Anandtech or Techreport I think), but I read a bench a couple months ago of the sis chipset compared to nForce and Via - it's a good chipset (in quality/performance of the 3, it's a close 2nd to nforce, IMO), but it has a 'cheap' reputation, so it gets the cheap treatment by mobo mfgs.
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