RG-58 will work well up past 450MHz. Depending on what manufacturer you look at, you'll see it spec'd up to 5gHz.
Of course performance is going to be pretty bad up that high.
As for 800MHz use, it'll be fine if your cable run is reasonable. I've got 40+ 800MHz mobiles at work and every one of them is using RG-58 coaxial cable between the radio and the NMO mount.
While it's not ideal, it'll work. For comparison…
10 feet of RG-58 will have 1.4db at 800MHz.
10 feet of LMR-240 will have 0.7db at 800MHz.
You won't hear that 0.7dB of difference in most cases, maybe at the fringes of coverage. Still, the price difference isn't much. Personally I'd probably go for the LMR-240 for my own use. At work, I'd have no problem using RG-58. Our system is designed well enough that 0.7dB of extra loss is going to break the system.
One thing to consider, though, is that LMR-240 has a solid center conductor, RG-58 has a stranded. It's easier to run RG-58 in a vehicle, although 240 is not impossible, it's just stiffer.
And, as others have said, You won't find RG-6 in a premade NMO mount kit. No reason for manufacturers to use a 75Ω cable in an environment that traditionally uses 50Ω.
For reference, 10 feet of RG-6 at 800MHz is going to have .87dB of loss. Better than RG-58, but again, you'll probably not be able to notice the difference.