I looked up an article
on anandtech to see if anything has changed since i got my g.skill HZ kit.
It looks like corsiar's xms2 ddr2-800 beats the HZ's, but if you notice the model number is CM2X1024-6400C3
and i can only find C4 on newegg. There might be a difference between the C3 and C4 models.
The g.skill HZ's are still a good choice for a high end system. Besides the xms2, g.skill HZ's is holding its own against the ddr2-1000+ ram kits (except at the higher memory speeds since it is rated for ddr2 800). It also gets up to ddr2-1067 for overclocking which would give your friend a nice overclocked conroe if he has sufficient cooling or justs wants a higher memory band width with a low cpu:memory divider.
As far as price/performance, I would have to say the corsair xms2's are the best since they are $23 less then the HZ's and they have a pretty good mail-in-rebate at the moment (well actually it seems they always have a nice rebate as far as i can remember them coming out