When buying a computer for Ubuntu, what should I look for? [duplicate]

I am buying a computer with the main intent of running Ubuntu, would I get the best compatibility and performance with a Windows based computer or a Mac based computer?

If it runs better on Windows based machines:

• Which manufacturer (Dell, HP, ...) has the best compatibility with Ubuntu?
• Does Ubuntu run on the Intel iX series as well as it does on the AMD series?
• Are there any problems running Ubuntu on an AMD processor?

If it runs better on Mac based machines:

• Does Ubuntu support Thunderbolt? Will it support Thunderbolt in 13.10?
• How good is the support for the hardware peripherals? Network Card, Graphics Card, ...

General questions about both:

• Are SSD's recommended or not recommended? Should you not use a Swap partition with an SSD?
• What is the maximum amount of RAM Ubuntu-64 bit can handle?
• Does Ubuntu have support for quad+ core processors?

Edit:

I am interested in getting a Desktop (so if it were a Mac it would be the iMac). I would want to spend less than $1500 US.