See now that's what makes prebuilt computers funny these days - they all come with i7's.
For anything but CAD/Photoshop/etc you don't need an i7, as an i5 is actually quicker in single-threaded apps (most games are included in this), but they still choose to add them in (instead of upgrading the GPU which is what they should be doing!). And while there isn't that much margin on prebuilt vs homebuilt anymore (or so you think), you are paying for lower quality components that actually have quite a high difference in price if you were to spec them. A pet hate of mine are prebuilds for the pure fact they're always cheap-ass power supplies, destined to blow up in an instant.
And as above, if you're doing CAD, you really need a Quadro card.