I moved this to Off-Topic; the PC Support section here is for Terraria-related issues only.
IMO, it's better to look at the recommended specs for a number of new games that you might want to play and use that as the basis for your choice. I'm sure there are a lot of computers you could buy for US$1,800 (or even less) that would play AC:O just fine,
according to the recommended specs Ubisoft gives. Giving an answer to "which is the best" is not really possible, there are likely many essentially equivalent choices.
If looking at hardware specs and configurations makes your head spin, you're probably better off going to your local Best Buy or independent computer shop and talking to them about what you want and need in a computer, and they will point you in the right general direction. You might pay a little (or a lot) more for someone's expertise that way. Just make sure that if you buy a custom build from a dealer that you get the license info for the Windows OS with it - some dealers have been known to use the same install disks for multiple builds, and then when something goes wrong you either have to go back to the dealer or you're out of luck.
If you're at all willing to learn a bit and think about building your own, I can personally say there's nothing more rewarding than doing that. You'll get more for your money that way, you get exactly what you want (and nothing you don't), and you'll be able to upgrade it yourself as time passes. If you can replace or add a disk drive or memory, or replace a power supply, you can build your own. It sounds daunting at first, but it's really not that hard, and there are lots of on-line resources to help.
Tom's Hardware: This entire site is geared to learning about and comparing PC components and builds. As this page shows, $1800 would build a pretty sweet rig. There is an active forum there with expert advice - I'll venture to say much better advice than you'll get from this forum, in general.
PCPartPicker: This site lets you virtually assemble a computer from parts, it checks compatibility between the components, gives approximate prices and various on-line sources for them, checks for rebate offers, etc. You can look at builds that other people made and posted, and see their comments about them.
There are a lot more than these, but I used these two sites extensively to research and build my own 4 years ago, and I couldn't be happier with the result.
Your problem isn' your hardware... as far as origins is concerned. Ubisoft has drm protecting their drm and it's eating up the cpu on high end rigs and straight up blue screening average rigs after a few hours of play.
The real question is why are you paying for ubisoft games? If you want to play with garbage that badly then consider going dumpster diving. At least that way you'll spend less money and might actually find some cool stuff.
While your opinion of Ubisoft and DRM has some valid points, it's really not helping him with his question.
