The Steam Deck's GPU could probably handle running most PS2/GameCube emulation at a much higher resolution for added sharpness, but on this size screen I doubt I'd notice a substantial difference, and I'd just be burning battery life by making the GPU work harder. In Dolphin and PCSX2 I'm running at 2X native resolution with added anti-aliasing and anisotropic filtering, which means games are running at about the native resolution of the 1280x800 display. The Steam Deck's screen size and resolution is perfect for those games. Increased resolutions can break the cohesion of a game's art style, sometimes giving you super sharp 3D against blurry 2D backgrounds, for example. Many old games can't upscale that well, though. This from a system that could only render at 480p!
I took these screenshots via Dolphin on my PC years ago. There are some retro games I love emulating on my PC and rendering at 4K because their graphics hold up so incredibly well-look at how amazing the Wii's Super Mario Galaxy 2 and Zelda: Skyward Sword are at hi-res, for example. My emulation gaming experience has been great so far Easily fixed: Under Config > Interface Settings, check the "Keep Window on Top" box for the render window. In Dolphin, I ran into an issue where the game render window and the Dolphin menu window were fighting each other for control, leading to a really annoying flickering problem. One other thing you'll need to do in each emulator is make sure that games are set to open in fullscreen. If you want hotkeys for saving and loading savestates to multiple slots, for example, that's doable with a little extra work. If you want to get fancy you can add a keyboard binding to the last open back grip and the left trackpad, or even configure an "action set" where you hold one button down to give a whole bunch of other buttons secondary keybinds. And this way there's no conflict with the controller bindings you'll want to set for each emulator. This lets you use the right trackpad as a mouse and the grip buttons on the back of the Steam Deck as your mouse clicks for navigating the emulator menus.
Because of the way Valve set up the partitions and file permissions on the Deck, installing emulators or any other software via the terminal is pretty locked down unless you have some serious Linux skills.
The best bit is that these aren't just one-off executables you're installing-when new builds of the emulators are released, you can update with a single click, too.
There are some other emulators available in Discover, too, including mGBA for the Game Boy Advance and Citra for the 3DS. bsnes (opens in new tab), for the Super Nintendo.Yuzu (opens in new tab), for Nintendo Switch.