Overclocking can't do miracles. It's a good result to squeeze 3GHz out of a Brisbane-Athlon but the resulting 20% perfomance increase are just on the edge of noticeability. There's no way it can compete with a Phenom II at any rate.
If she can get the cpu up to 3ghz it will be noticeable; I have yet to see any evidence sims 3 fully utilizes quad cores preventing maximized returns on phenom II.
And the graphics card won't be a bottleneck, TS3 ist just as CPU-limited as TS2.
There's no such thing as a general bottleneck, there are limits, and it really depends on the game you're playing. Unless you don't go and try to play TS3 on an IGP or something it's going to be CPU-limited no matter what.
In the context of sims 2/sims 3 this is true, but Skadi purchase a gt250 which is overkill for the sims 3, I made an assumption that she also plays other games where her current cpu will bottleneck; she has acknowledge this herself.
Whoever said that overclocking would be good needs to google
because overclocking is usually something that many try not to do. Overclocking a card will only give a small gain but also give a pretty big chance of a faster death, of either being burnt out or the person clocking it to high and then, well burning it out XD. Leave overclocking to the maker's of the card, Sim fans aren't the kind of gamers who should even have to bother with overclocking, because if you can't play the game fine it's not going to get any better just because you overclocked a crappy card or processor.
Your advice is sound for the "average" sims player who purchase their computers off dell.com but both Luisa and Skadi and stated in their posts they built their computers themselves, thus are already quite tech savy and capable of researching and deciding whether it a good idea.
If you could only upgrade one thing, I'd go for maxing out the RAM. But since you will probably find out you need a new motherboard to do that effectively, you'll probably end up having to upgrade everything, cheap.
A few weeks ago when TS3 came out, I bookmarked some cheap upgrade items on Newegg. I bookmarked this ASUS motherboard ($69.99) with dual core Phenom II cpu ($102.99) as a reasonable starting point. It has room for 16gb ram, although 8 would be more than enough to last until the next upgrade.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131381http://www.newegg.com/Product/Product.aspx?Item=N82E16819103680Luisa current gigabyte mobo supports up to 16gb of ram spread over 4 mem slots; Luisa is based in England and newegg does not ship internationally.