Last night I stayed up a bit too late playing GTA SA on the highest possible graphical settings, and it was... amazing! Toggling between the different texture qualities didn't seem to make that big of a difference back when I was playing the game with my old computer, but there's more than texture quality to toggle in order to increase the quality of the game.
The most important feature is definitely the Frame Limiter. All the old GTA games, up to the third generation, relied on a limited amount of FPS to run properly on the limited gaming hardware in that time. By default, the frame limiter is thus turned on, limiting the amount of frames generated per second, but you can turn it off for all third generation games. In GTA 3 and VC it resulted in weird physical abnormalities, like people flying up into the air when you hit them, etc, but since I've always played those games with the frame limiter turned off anyway I just thought it was the game engine at that time being a bit buggy. With GTA SA, however, my old graphic card wouldn't let me adjust this setting with resulting lag, making it seem like I hadn't turned off the frame limiter at all. I tried it again now, and it's incredibly how smooth the game is to play. It's a whole different dimension. Driving around, walking around, everything you do no longer clips. The clipping is something you might think to attribute to lag, but it's actually just the limited FPS on the game. So changing this one option has the biggest impact, and if your computer isn't too old or scrawny, it'll be a huge overhaul.
After that, there's anti-aliasing, which adds outlines to all shapes, making everything seem clear and defined. This is another simple change that enhances the graphics more than you'd expect, and also an option I wasn't able to try out on my older computer. You can set the thickness of these lines as much as you like (I set it to 1). On newer games there usually isn't even an option for single anti-aliasing, it's used in multiples of two, making the lines seem far too bulky in my opinion. Or maybe it's the fact that modern models don't really need outlines to look good, I don't know, but for this game it works wonders.
The third most important option is the screen resolution, setting it as high as the monitor allows adds a lot of clarity to the image. I played GTA VC a few months ago on 640x480px since I didn't know how to change the resolution (protip: it can only be changed before you load/start a game), and when I finally did change it, the difference was gargantuan. If you can see the pixels on the screen when you look close, you're probably not using a big enough resolution. So, with this maxed out, anti-aliasing on, and frame limiter off, there are just two more options: draw distance and texture quality.
The draw distance specifies how far you can see in the game, IOW how much of the surrounding graphics are rendered as you look around. If you notice textures disappearing or taking long to load, chances are you have the draw distance set past the limitations of your VRAM (graphic card memory). I've always had a bit of a problem with disappearing textures, even on a vanilla install, but with the new GPU this did not seem to affect gameplay at all. When textures now take a bit longer to load I believe it's the games built-in limitations rather than the computer's limitations at play, so, maybe there is a script to adjust the object limitations within the game. Hmm..
Oh, texture quality. I'm not sure how this works, if textures are compressed, or if there are different resolutions, or what it is... but this is the one option that seems to affect your machines performance the least out of the ones mentioned. Jumping between low and very-high does produce a significant level of difference in detail, but for each step the differences are marginal. I haven't been able to max this out earlier without lag, but this time around I'm playing without limits!
SRT3 and the LOD MOD are up next... and maybe ENB Series if I can adjust the settings for reflection and light without using too much bloom. Most of the videos I watch of people recording ENB footage has waaay too much bloom, the world is too bright and diffuse, it's like a dreamworld. I mean, it's too fuzzy. Btw, if you want to use ENB but it's too high-end for you, try Ultrathing, it's a great low-end alternative that produces many similar effects with much less resource use. That's all for now, when I do have the game fully tweaked/modded later on, I'll post... a post. Later.