PC spec for retrogames

You will definitely need a PC for PS3 and 360 emulation. The website for RPCS3 and Xenia should have the system requirements for the emulators that will be a good place to start.

If you want to emulate those systems, then yes, you're going to need a fairly beefy computer. You could, as others have suggested, buy a good secondhand system and upgrade it with a GPU and more/better RAM.

But I want to pass on a warning as someone who also loves emulation and wishes they could "have everything in one place": a lot of emulators just aren't there yet, but some people are eager to kid themselves and others that they are.

16-bit systems and before typically have outstanding emulators available. Some systems from the next couple of generations are also very reliable (e.g. PS1, Dreamcast), while others mostly work well with minimal tinkering and only a small handful of exceptions (e.g. N64, Saturn). But after those, the reliability of emulators drops off fairly smoothly. Even the venerable PCSX2, for example, will run almost every known PS2 game in some fashion, but many games outside the biggest hits still have problems that make them terrible. And I don't mean picky things like, "Three notes in the bassline on this background music are slightly off," I mean, "The walls aren't rendered in most areas."

I really recommend having a good look at the compatibility lists for emulators you're interested in before you dive too deep down this hole. It's one thing to have a powerful PC already and think, "why not give it a go?" but another thing to build a new (to you) PC specifically for emulating these systems. I suspect that you may have been spoiled a bit by that fact that even the RP4 only has enough power to run those more stable emulators for older systems.

Probably best find the docs for the emulators you're planning on using and see if they have any minimum or recommended specs listed. I know a lot of emulators have really come a long way in the last few years some of them might support really low-powered machines.

Anecdotally, I recently built a PC out of old bits and leftovers from friends' upgrades. I used it more for native PC games but it was an incredibly cheap way of playing anything that wasn't a recent AAA resource hog, so maybe see if people you know have parts that are just gathering dust.

If you are comfortable with building a pc by yourself, you could try to get an older office pc and put a decent graphics card in there.

Why not just get the actual PS3 and 360? They're pretty cheap nowadays, both have HDMI output, and PS3 is easy to softmod

It's quite a significant step up in hardware.

I've not gone as far as PS3 but for PS2 emulation I had an old PC with an i5 4670, GTX970 and 8GB DDR3 RAM and it worked well.

Id recommend an Intel NUC, but Intel isnt really making them anymore and the old ones have for some reason jumped in price drastically. But even the old NUC7s can go all the way up to PS3 and X360 technically, but unless its a simple game there will be slowdowns. Demons Souls runs at about 20fps at 720p, for example.

However, a PC that you're building can have the following specs that will emulate everything currently available:

intel i5 12000 or higher CPU

nVidia RTX 2070 Super or higher GPU

16Gb RAM

Its possible that you might be able to squeak by with a GTX 1050Ti GPU, but I cannot confirm if that is true.

Steam Deck