A single 7800GTX is actually almost identical to RSX, not vastly superior. In fact early on 7800GTX was clocked at 430mhz. It may have had more bandwidth, but when you combine PS3's two buses with the ability of programmers to use tricks in a closed box console enviroment, the 7800GTX's bandwidth advantage was probably not a factor in real terms.
I was talking about the 7800GTX 512MB, not the standard version. If anything the standard version may be a little slower than RSX, at least when you cosider the memory bandwidth to XDR.
However the 512MB is a different story. I'm certainly not saying its vastly superior to RSX, just thats its faster by a noticable degree.
It has a 10% more clock speed in the core, twice as many ROPs (thus more than double the fill rate), a dedicated pool of 512MB just for graphics and more memory bandwidth than PS3 has to share between both RSX and Cell.
Whoa that's way wrong, lets assume it's about as capable as RSX (most games show this) then it's about as capable as a 7800GTX which puts it firmly in X1800/X1900 class.
The X1800 was quite a bit more capable than the original 7800GTX (which I believe is probably a closer analogy to RSX than the GTX 512MB) and the X1900 is faster again. In fact your looking at a 25-50% speed advantage of the X1900 over a vanilla 7800GTX depending on how modern the game is (the advantage is greater in newer games).
Anyway, HD2600XT + 25% is roughly in 7800GTX territory. In fact its probably pushing 7900GTX territory and in some modern games even more.
That, plus the fact that on paper Xenos is almost exactly 125% the raw performance of the HD2600 (not taking into account any architectural improvements ATI may have introduced) makes it a pretty reasonable assumption in my view.
Looking back at it 360 did seem to be about as powerful GPU wise as the top PC's at the time. PS3 was a different story, as it came out a year later. Of course not counting SLI here, which is a bit of a "cheat" and rarely works well anyway.
I think saying it rarely works well is a bit unfair. At higher resolutions, even back then SLI scaled quite well on the big name games. I see your point about it being a bit of a cheat but at the end of the day it was an option that PC gamers could use and in so doing they could play games that were available on both systems at much higher settings than the 360 was capable of (Oblivions a good example).
Looking at just the one GPU then I agree Xenos was probably roughly on a par with a single 7800GTX 512MB. Each obviously has very different strengths through.