A cross-6th gen developer here on the b3d forums explained how much more powerfull the xbox was over its competitors. The PS2 had some fillrate advantage over the xbox though (but needed it aswell).
Its nowhere near what it is today where consoles are basically the same, 6th gen was the most intresting generation regarding hardware, games and innovations.
On the MGS2 xbox comment, its quite impressive considering that game was designed towards the PS2's strengths and it will ran quite well on the xbox with it having some advantages aswell, konami wasnt either well known for great ports.
The lack of the PS2 controller for MGS2 xbox was a bigger problem i think.
I've played the entire MGS2 on the Xbox,
for the most part it was fine, but there were some places were it ran really slow, the game speed was tied to framerate making it really odd to play it in proper slow motion later on, there was a part with a lot of smoke or water or something that was likely fill rate related and it struggled pretty badly
not that I blame the console, it was just no properly optimized for the hardware, but yeah, you could make PS2 games run poorly on the Xbox depending on their demands,
but realistically speaking doing the opposite (XB optimized title port to PS2) would be a far bigger challenge and they wouldn't be able to get away with the basic ps2 to xb ports they did for sure.
hypothetical 512MB for the XB is super unrealistic due to cost,
128MB less so, but I don't think that it would have changed the life the console had.