iMac Retina with another 4K monitor.
Have you seen the price of 4K displays???? This is the least expensive way to do it as per OP's request?
Have you seen the price of 4K displays?
HiThe old Mac Pro with a modern PC GPU.
If 2x 4K monitor is all you need regardless the processing power. Even the Mac Pro 1,1, can do that with a current PCIe graphic card (e.g. 750Ti). This setup can give you dual 4K output with less than $150 (of course, it will cost you some time to buy the cheap stuff on eBay).
A more make sense combination is something like the Mac Pro 4,1 with the Maxwell GPU, e.g. GTX 950.
Hi
Can a ATI Radeon HD 5770 video card handle one 4K screen. I already have the screens. They are TVs Seiki 39". I love them. They were on sale for $250.oo at New Egg. I bought them for $364 before that from Tiger Direct.
Yes and No. Yes, it can, but only at 30Hz, generally not a good idea for computer user. And you may need some 3rd party software to achieve that.
The Seiki TV's can only do 30hz anyways as well.