the main task is for my 8 y/o girl to play WoW and SC2 and Flash. and once in a blue moon play BF3, FC3, Crysis. and to surf the internet. and to watch youtube.
hyperthreading is moot. most game will not make good use of it.
Hyperthreading is moot when talking about 4C / 4T vs 4C / 8T CPUs. It is certainly NOT moot when talking about 2 core CPUs. SC2 and WoW, yes these are primarily 2 core games. The others WILL make use of hyperthreading on 2C / 4T CPUs even if they don't show improvements with 4C / 8T CPUs.
This is from my own experience using a 2C / 4T Clarkdale. Games definitely use the Hyperthreading, and it is definitely a huge difference in some games. No game machine should consider less than a 2C / 4T CPU IMO, even at the expense of lower IPC.
Clarkdale IPC is well below IB IPC, but the IPC of the IB Celerons are below that of the "core ix" CPUs that the reviewers give press to. They are cut down versions of the "press-worthy" CPUs. OCing makes up for any IPC differences. 4GHz is pretty much a given OC. 4.4 requires better cooling and more on the edge. 4GHz makes up for more than 20% IPC, and the HT puts it *well* ahead in any game that can make use of more than 2 cores. Comparing to an IB i3 is a much closer comparison, but comparing to non-HT IBs, no, I like the Clarkdale.
This is the closest "bench" comparison available:
http://www.anandtech.com/bench/Product/404?vs=145
Very close single threaded performance, maybe the SB Pentium has a slight advantage over the i5 Clarkdale, but anything that makes use of the hyperthreading pulls ahead. You see this in the gaming benchmarks too. WoW is almost dead even.
I haven't played WoW in over a year, but my i3 @ 4 GHz was fine with WoW. Certainly faster than what most people playing WoW have. In my WoW guild, I was considered some kind of hardware god with an OCed Clarkdale and SSD. You can't even imagine what kind of garbage hardware most people are playing WoW on.
There is certainly some value in using a platform that can be upgraded, The IB Pentiums are definitely going to be better for that aspect. I'd still hesitate to put anything less than 2C / 4T into a gaming machine, anything reasonably new is going to struggle with 2C / 2T. If you can stretch to an IB i3, then there's no question at all. If not overclocking, you can put that into a pretty low budget motherboard.