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@codelad In your experience with Pi, did a point ever come when you wished it had a more RAM? If 8 gigs of RAM would be an overkill and consequent increased cost would defeat the purpose of getting a low cost, low powered device, then which iteration of Pi would hit the proverbial sweet spot? Because as 8 GB version isn't so easily available even from authorized sellers, I have no issue in getting a pre-loved unit thus further driving my cost down.
Once you start pushing hard, you invariably run into several bottlenecks - not just the memory, but also with the CPU, I/O, thermal issues and stability. From running several Pi 1 devices with 512 MB memory, I have faced issues with running a DLNA server (anything above 720p just wouldn't play), and as a motion capture setup (when I tried to merge the images as a video file in the device, it just won't handle the load).
Obviously, you'll realise that you can work around some of these by adjusting your setup, and that with some, the Pi isn't going to cut it.
A more modern (and powerful) revision may resolve some of these hardware limitations, but I can't quite say for certain where the "sweet spot" may lie. I recently got a Pi 2 (with 1 GB memory), but haven't used it much.
All said, the Pi is a fun device and I run several of them. Once I'm able to set up a basic Arch Linux installation and get wireless configured, to the point where you can SSH into it - from there on, you have a neat little (largely self-contained) remote device. With a decent phone charger, you can basically plug it in anywhere and use it for anything. One of my favourite use cases is as a MPD server. It's quite easy to set up, and with enough storage attached - even something as simple as a USB thumb drive, I can stream music to any other device in the house (and control it from our phones, as there are neat Android MPD clients available).
If you don't necessarily need the latest and the greatest Pi revision or are unsure, it's not at all a bad idea to pick up an older used Pi. I've seen them sold here for below 1k, and you can get several for the price of one newer Pi (the way I look at it - YMMV).
@codelad true , initially raspberry pi etc looks exciting than you run services such nas etc than you feel its very under powered its sensible to jump on less power ful atx , even old atom boards would work well
While this is partly true, I personally don't always look at the Pi as something to get things done with. It's also a fun device - something you could experiment with and learn from, and try stuff that you won't (or can't) with a regular machine. And I think this is where the Pi (and similar boards) really shine.
For several years, I ran a very low-power Atom 230 (something I could keep powered on 24/7 should I need it), and at some point relegated some tasks to the Pis. And now (with my Atom 230 long gone), I do the same with a low-power AMD Hudson and the Pis. I say this, just to drive home my point that the Pi still has a place alongside regular hardware.