Overclocking both the 7800 GT and GTX is a little bit more complicated than usual. In practice It’s no harder than any other card, it’s just you have three clocks to deal with - the vertex, shader and ROP clocks. The slider you tweak in software overclocking utilities like ATI Tool or Riva Tuner actually adjusts something known as a root clock.

Traditionally for a stock 7800 GT you would have a root clock of 400 MHz. This does not mean the core is clocked at 400 MHz as the core is made up of different processing units which as already noted above have independent clocks.

There is no easy was of explaining this, but the important thing to remember is not that there are three clocks all running at different speeds, but that some of them do not increase on adjusting the root clock until you reach a specific oscillation.

You’ll probably need some more details to understand this fully:

• The vertex clock is always 40 MHz higher than the root clock.
• The shader and ROP clocks increase every time you hit a 27 MHz oscillation of the vertex clock (a vertex clock of 486 MHz is 18x27 and so the shader and ROP clocks will increase).
• When the vertex clock hits a 27 MHz oscillation the shader and ROP clocks will be 27MHz less than the root clock when the oscillation takes place. The shader and ROP clocks will not increase until the next 27 MHz oscillation of the vertex clock occurs. Another way to calculate shader and ROP clocks would be that both of them will be 13.5 MHz higher than the root clock.

For example, if you select a root clock of 445 MHz you will have a vertex clock of 485 MHz and the shader and ROP clocks will be 432 MHz.

Why will the shader and ROP clocks be 432 MHz? Because the last 27 MHz oscillation of the vertex clock took place at 459 MHz (27x17). If we now take what we know, which is that the shader and ROP clocks will always be 13.5 MHz higher than the vertex clock was at its last oscillation we can easily predict when the shader and ROP clocks will make a jump.

If you now change the root clock to 446 MHz you will get a vertex clock of 486 MHz, which is a magic 27 MHz vertex oscillation (18x27=486), so the shader and ROP Clocks will increase to 459 MHz (13.5Mhz higher than the root clock, which is 446 Mhz in this example).

This should explain why the performance difference between 445 MHz and 446 MHz is substantial on a 7800 GT. The shader and ROP clocks increased 27 MHz by increasing the root clock by 1 MHz.

Below is a table covering most realistic overclocks with a 7800 GT to help you aim for the right settings:

From this table you can clearly see that the shader and ROP clocks are handled by a 27 MHz oscillation. Never set your overclock just below an oscillation as you will miss out on the shader and ROP clock increases, and that’s just a plain stupid thing to do. This also explains why, for example, some people can wit a root clock of 445 all day but 446 cause’s instability problems. It’s not a 1 MHz increase like it appears on the surface.

With that out of the way I have to now break it to you that this isn’t how this 7800 GT work! The Gainward BLISS 7800 GT GLH just overclocks normally. If you set a root clock of 450 MHz, everything else will be 450 MHz. As the card at stock runs at 450 MHz you actually loose performance over other 7800 GT’s.

Though you can’t dispute the evidence here, this card is extremely fast as you’ll see shortly. The 1.3 GHz clock speeds on this card must be attributed to this. Let’s move onto how this card overclocked.

A standard 7800 GT is clocked at 400/1000, but Gainward’s card is clocked at 450/1300, which easily makes it the fastest 7800 GT on the planet.

I was able to push the core another 30 MHz before I hit problems. This is the highest overclock I’ve managed on a GT without volt modding. Don’t forget that this is 480 MHz on all clocks (root, vertex, shader and ROP). If you missed that part jump up a few paragraphs.

Obviously the interesting bit here is the memory. No other 7800 GT comes with memory as ‘big daddy’ as this. I was able to reach a massive 736 MHz, which is pretty much the same overclock as reached on the 6800 GS GLH. It’s a slight indication of what to expect from the ‘Goes like Hell’ range of cards.