Yes, you would and most certainly do. You are mistakingly taking the term "pixels" to mean only the rectangular (in the case of the 4500) RGB plasma cells of the screen. The term "pixel" actually means picture element and can consist of literally anything making up the composition of a given picture. When speaking of being able to see pixels in 240p and 480p games from 5 feet away, regardless if your display is 720p, 1080p, or even 2160p in the case of 4K displays, Im speaking of the pixels drawn by the input device (in this case, an older video game system). Regardless of upscaling, a pixel of a 320x240 image is exactly 4 times larger than a pixel of a 640x480 image, which is exactly 3 times larger than a pixel of a 1280x720 image and it is of this type of pixel I that am describing.
When the set upscales a very low res image, it lights multiple plasma cells to make up a single pixel of the source image. In the case of a 240p game system like the N64, the pixels drawn and sent by the N64 to the set are almost 16 times larger than the plasma cells of the 720p screen (not exactly though, because the screen is 1024x768). You can most certainly see these pixels regardless of the resolution of your set. They actually are even more pronounced on higher res displays.
Interesting discussion! I also play a lot of classic consoles and for display discussion, the scaling element interests me the most. You're right. If we're to blow up a 320x240 image to a 1080p display, we quadrapple the pixels to 1280x960 resolutions with a few black bars on all sides to spare. As you said, this would blow up a pixel to be 16 pixels, (4x4) making a pixel painfully obvious to see. This may not be as problematic if we're to only dealing with horizontal or vertical lines, but if we're talking about diagonals, it would have severe stairstepping that's not present on original source, and this is why so many classic games look crap when played on a modern HDTV. To make things worse, the actual scaling procedure from majority of TVs does not use line doubling much, so the 1280x960 resolution example I gave doesn't even apply. it will fill whole 1080 vertical resolution and now we won't even get 1:1 scaling. One way to mitigrate this is to use 1:1 pixel matching, but this is only useful for 720p Xbox 360/PS3 games. 240p games will look like a postage stamp on a 1080p display! Other method is to use interpolation which majority of TVs do use, but at the expense of making picture softer because now we're calculating a intermediate pixel between two original pixels and this will go on for 3 more times for 1080p displays. Some interpolations are better than others and those offered by PC emulators (2xSAI etc..) but even those are not perfect. (SNES would work better with this than Sega Genesis for example because of more onscreen colors ready to be used for interpolation)
It's very easy to see how scaling is destroying PQ on my Sony Trinitron BVM. This display accepts both 240p and 480i, and no, they are not the same. 240p games will leave out scanlines while 480i games will actually show those extra lines. Problem is, when a game that originally had 240p resolution is upconverted to 480i by game publishers to minimize problems when running on HDTVs, (I've tested this with my Silicon Ops video processors as well. 240p games gives heavy flickers) it will look like linedoubled garbage when running on 480i instead of 240p. (I call this effect "The emulator smell" btw because it does reek feeling of playing on emulators lol) 480p Dreamcast SNK fighters now will make full use of 480i resolutions on my BVM yet they're actually LESS sharp because of interpolation involved. (BTW, My BVM is awesome on interlace signals. I was shocked the detail loss caused by interlacing is actually minimal. They don't actually look too different from my other progressive scan enabled Sony CRT, the FW900) So, in a nutshell, a 2D game that's meant to be 240p that is blown to 480p will lack the crisp sharpless, will have more dull color, have more jaggies. If I were to feed Guilty Gear X on my BVM, it will still have problems associate with interlacing, but at least there won't be any more scaling problems because GGX has true 640x480 resolution. (24hz games like Street Fighter 3 is another story. It's higher than 240p, but lower than 480p) Right now, the biggest 2D gaming casualty on my Panasonic 1080p plasma is King of Fighters XII and XIII because it acually mixes in TWO 2D resolutions in one. Charater sprites are 480p, yet the background is 720, making it impossible to pixel match on any displays.