As Charles_n_Town said, use the minverchk.pup to verify the minimum firmware on your console.
It should be 3.56 or lower in order to install Evilnat and OCing.
As for the OC frequencies possible for a 25XX, they all support at least 700MHz core and 850MHz VRAM, even the "weakest" RSX revision you can find in them, aka "
CXD5300A1GB".
The better chip available in an OCable 25XX will be the "
CXD5300GGB", which support at least 850MHz core (900MHz is quite frequent, 950MHz is possible but extremely rare) and up to 1000MHz VRAM.
There's also the
CXD5300CGB (the chip I have in my main Slim), able to support up to 850/1000 if you're lucky. Otherwise, 800/950 is easy for them.
The "
CXD5300DGB" is a pretty rare revision it seems, performing almost like a "
CXD5300A1GB".
With this one, forget about 800MHz core.
"
A1GB" and "
DGB" may hold 750MHz core stable, but don't bet on it.
There's always small variations, even between two same chips because of different manufacturers, but you'll never see a "GGB" unable to sustain 800MHz core, for example. And you'll never see a 25XX not supporting 700/850.
The RSX revision is the main factor determining the overclock potential.
Keep in mind that overclocking is always risky !
Never jump to high values directly, even with a GGB.
Having an e3flasher is recommended.
If you're wondering how to find your RSX revision, you'll have to check your RSX's IHS before applying thermal paste. That's the only way to be sure.