I've been looking back at the specs of legacy consoles of late just out of interest and found it difficult to find accurate information on the OG XBOX.

While it's true that the numbers vary depending on your sources it seems widely agreed upon that when it comes to the performance of consoles in the sixth generation the floating point performance of the Dreamcast was 1.4 GFLOPS, the PS2 was 6.2 GFLOPS and the GameCube was 9.4 GFLOPS. As for the XBOX, well the wiki tech specs state the NVidia GPU has a floating point performance of 7.3 GFLOPS. Now I know that when it comes to overall graphical fidelity and performance there is way more that you need to take into account than just FLOPS such number of shaders, memory bandwidth, amount of RAM etc but I find it hard to believe that the XBOX comes in less than the GameCube in this area. I always thought the XBOX was a beast compared to the competition. 

Another article I found puts the XBOX at 20 GFLOPS but this seems too high and potentially inaccurate.

https://www.gamespot.com/articles/console-specs-compared-xbox-one-x-ps4-pro-switch-a/1100-6443665/

I was wondering if anyone here on VG Chartz with better technical knowledge then myself can clarify this for me. Was the XBOX the graphical powerhouse I thought it was or is the GameCube actually more powerful?

Your best bet is Pemalite. I would @ to summon him, but I've already bothered him this week. Gotta space these things out.

The Original Xbox is a 5.8Gflop GPU machine.

But like you said, there is far more to a GPU's capability than mere Flops.

The Original Xbox simply uses a far more efficient GPU architecture than the other consoles of that generation, so it was able to punch well above it's implied weight. (I.E. Better culling, compression, programmable shaders and so on.)

The Gamecube however was a texturing monster, it's TEV was also pretty capable.

All I  can remember is the GC having the sweetest looking water in games. But I seriously doubt the GameCube could run Halo 2, Dead or Alive, Half Life 2, Forza, Ninja Gaiden, etc. The Xbox was a beast. I must mention here though that RE4 in the GC totally murders the PS2 version graphically. I bought the PS2 version for the extra content but it was a step down from the gorgeous GC version.

Game Cube to me always has overall better looking games. I never owned a OG Xbox but from what i saw, the Xbox lacked effects that the GameCube could dish out. From memory i believed the Cube could render 8 different effects per ploygon compared to the PS2s 2 effects per polygon. Not sure what the Xbox was doing but i was also believed it was less, around the 4 or 6 mark.

The Original Xbox is able to have better shadered water than the Gamecube... But the Gamecube can have better textured water.

However... To be fair, the Gamecube is technically capable of every graphics effect that the Original Xbox is capable of, it just requires additional passes or workarounds in order to achieve it... Which let's face it, generally never happened until the Wii came along anyway and developers had years more to extract from it's similar architecture.

That generally stems from Nintendo's pretty talented art direction.
But from a technical perspective, Original Xbox games were the best of that console generation.

The Original Xbox also had a pretty potent CPU as well... Which meant that Physics started to become more prominent in games. (Half Life 2 for example)
And the GPU was such a big step up that there were a handful of games that operated in High Definition.

I always thought it was the other way around with xbox's shader pipeline allowing for direct PC ports. Gamecube was great hardware that excelled in many areas though. The TEV fixed function shaders were great at what they could do though.  

The Xbox's GPU AND CPU was PC derived, hence why it garnered so many PC ports and why it was so easy.

The graphics chips from that console generation really did have their own unique sets of strengths and weaknesses. The PS2's GPU for instance was able to cope with insanely high polygon counts for the time, but the moment you asked it to do anything other than generate tons of polygons, performance would tank to sub-Dreamcast levels. Similarly, the Dreamcast's own GPU had certain features that the other companies wouldn't incorporate until the PS4/Xbox One generation; it's just that it wasn't powerful enough to do much useful with them.

Thanks to all above for the information!
The sixth generation is so interesting as consoles that were in direct competition were so fundamentally different in their architecture. I think Pemalite's point about the XBOX having a superior CPU and therefore able to handle games with advanced physics is incredibly important too.
At the end of the day the more I read about the GameCube the more I like it....