So could HiRez downloads sound as good as SACD?

In short, yes. Yes HiRez can match SACD sound quality and yes you're wrong thinking that the "lossless stuff" in only for redbook CD.
Any computer with USB and a (preferably asynchronous and capable of more than 16/44) DAC will give you hirez audio playback.
Asynchronous USB is the best way to go computer-based, but it's not the only way.
New universal players, like OPPO BDP-95, can play hi-rez files directly from a hard drive, no computer needed. Since it also plays SACD, it will give you a good way to compare the SACD and hirez formats alone while keeping everything else the same.

Thanks for the lesson Mihaitaa!

High res digital has a few different word lengths and sampling rates. Cd is 16/44.1. High res can be 24/88.2, 24/176.4, 24/96, 24/192.

You need a DAC that can convert the different word length/sample rates to analog. There are many DACs available that can do this.

After that the choices and complexity or simplicity of the system grow. There are many different ways to store the music and get it to your DAC. I would do quite a bit of reading before you try putting a system together.

it is possible that for a particular digital front end redbook sounds better than sacd using another particular front end.

i have hirez dvrs, from refereence recordings, and i prefer their sound to sacd . but maybe i don't Have the best sacd player.

so it seems that the answer is a combination of software and hardware