Well, it really comes down to 1) do you want to play in the game's native resolution or 2) play the game in HD resolutions like 720p, 1080p or 1440p or 3) play the game in "ridiculous" resolutions. For many games, a GTX750 Ti will have no problem doing 3x IR and 4x IR (depending on the game of course) in Dolphin (both IRs are larger than 1080p actually) and even though I've had limited experience in PCSX2, I can't imagine it struggling for 1080p+ gameplay on that emulator either, not until you start setting it to those "ridiculous" resolutions.
Anti-aliasing depends on how sensitive you are to "jaggies". At higher resolutions, aliasing gets less and less noticeable, but most people can still see it, even at 1080p+, hence the need for AA. You really shouldn't have any trouble doing 3x IR + 4xSSAA in many games in Dolphin, can't say for sure about PCSX2. MSAA is even cheaper in terms of using GPU resources, so if you can deal with only geometry (that is to say polygonal) anti-aliasing, you can often go 3x IR + 8xMSAA all the way to as high as you can set it, depending on the game of course. GPU intensive games will require you to scale back, but with a 750 Ti, you should be able to set it fairly high in many games. Obviously when talking about N64, PSX, and DC games, you should be able to max it out as much as you want; they're not nearly going to be as demanding as Dolphin or PCSX2.
As for how different an emulators HD output is from a console, just take a look at comparisons online. The main site has plenty of HD Dolphin screenshots to give you an idea of what 1080p gameplay looks like. The level of detail is orders beyond the original consoles in many cases. N64, PSX, and some DC games might not seem as impressive due to the models and textures used, but you'll still have more pixel data and higher image quality compared to the consoles themselves. As to if you even want to mess around with these graphical enhancements, that's for your preference to decide. Some people love HD emulation; others like to mimic the console as closely as possible.
Anti-aliasing depends on how sensitive you are to "jaggies". At higher resolutions, aliasing gets less and less noticeable, but most people can still see it, even at 1080p+, hence the need for AA. You really shouldn't have any trouble doing 3x IR + 4xSSAA in many games in Dolphin, can't say for sure about PCSX2. MSAA is even cheaper in terms of using GPU resources, so if you can deal with only geometry (that is to say polygonal) anti-aliasing, you can often go 3x IR + 8xMSAA all the way to as high as you can set it, depending on the game of course. GPU intensive games will require you to scale back, but with a 750 Ti, you should be able to set it fairly high in many games. Obviously when talking about N64, PSX, and DC games, you should be able to max it out as much as you want; they're not nearly going to be as demanding as Dolphin or PCSX2.
As for how different an emulators HD output is from a console, just take a look at comparisons online. The main site has plenty of HD Dolphin screenshots to give you an idea of what 1080p gameplay looks like. The level of detail is orders beyond the original consoles in many cases. N64, PSX, and some DC games might not seem as impressive due to the models and textures used, but you'll still have more pixel data and higher image quality compared to the consoles themselves. As to if you even want to mess around with these graphical enhancements, that's for your preference to decide. Some people love HD emulation; others like to mimic the console as closely as possible.