Most sites have a "PC Builder" option that allow you to pick your components and then they assemble them for a small fee.
I can't comment on ibuypower.com but regarding i5-3450 vs. i5-3570k basically what you pay for is what you get. The 3570k performs slightly better and is overclockable but also costs about 40 bucks more. The 3450 has very limited OC abilities and offer slightly less performance but is significantly cheaper.
Both are very good though and you won't go wrong with either.
I cannot recommend any of the two PCs you have listed. The Acer setup has NO discrete graphics card - it only has the Intel integrated graphics which do NOT perform well in games at all. Chances are actually somewhat good that your current graphics card will perform better, despite being much, much older. And in any case, the graphics in the Acer setup will not run Guild Wars 2 at reasonable framerates.
If people have bad experiences with that Ibuypower-brand, I would try to find something else. If you can manage to assemble the PC by yourself (or perhaps you have a more tech-savvy friend?), you can save a lot of money.
The bottom line of the two builds you listed is this: None of them will run GW2 very well. You can check this article on Toms Hardware to see how some different graphics cards perform in GW2: tomshardware.com/reviews/guild-wars-2-performance-benchmark,3268.html
Last edited by DarkwingDuck; 2012-08-27 at 08:56 PM.
Reason: Clear up point