Slickdeals.net has a nice deal on an Alienware Alpha Compact Desktop. This is the same computer as the $499 machine Alienware sells on their Zwift page, except with the Slickdeals coupon code knocking off $200, plus a $45 mail-in rebate, your total cost (including shipping) comes to $305!
It really works–a local friend of mine bought one last night.
Note: I haven’t personally tested this machine, but it should be able to run Zwift in ultra mode just fine. Again, this is the same machine Alienware recommends on their Zwift page.