I think likely improvements if the 360 had been designed to launch a year later...
Higher cpu clock speed. Possibly OOE. Had they implemented OOE, I'd expect Xenon to be showing higher real world performance then Cell in most situations, but then it's likely IBM could have used the same design expertise to give at least the main core of Cell OOE as well. Xenon and Cell lack OOE because it wasn't a priority for IBM, not because Microsoft wouldn't have liked it within their $100 million design budget. IF OOE had made it in, I think you would have been looking at a processor under 3ghz due to the additional heat concerns and design complexity. It may have also had to drop down to dual core to save die space. IMO, it'd still be superior design since it'd be far easier (possible) to get better results out of, though only if they could do it without ballooning the design budget. I'd expect 50%-100% more performance per core even at the lower clock which would more than offset the loss of a difficult to use additional core.
-Put it this way, Rogue Leader on Gamecube visually outperformed Halo, and I'd say Rebel Strike also outperformed nearly anything on Xbox. But the average xbox game still blew away the average Cube game. The most common result is the result people see and more important than a 1 time one-up over the competition.
Bigger edram.
The 64 unified shaders already in xenos wouldn't be downgraded to 48 for yield concerns.
Small chance of more system ram.
Harddrive would have been standard.
If a two sku system was still used, the higher sku may have included an hd-dvd drive and been priced at $500 to $600.
Oh, and as for the current Xbox 360 versus PS3, I'd say a fully optimized ps3 game will put out better screenshots, but if the xbox 360 can really do 4xAA at almost no performance hit, it will put out the better looking games in actual motion. Additionally, any advantage ps3 will have in graphics will be so small that it will be hard to tell for most people anyway, whereas the lack of jaggies on the 360 would be clearly noticeable. Ps3 will likely have more good looking exclusives though, microsoft draws too much on the PC game world to get exclusives, and that's really the only way they've been able to launch either one of their consoles with good game support.
PS2 did have a CPU and bandwitdh advantage over the xbox, but overall it suffered because of less ram and older GPU. However, this is not to imply that PS3 would suffer the same disadvantage.
PS2 had a flops advantage, I wouldn't say it necessarily had a cpu advantage. Additionally, the xbox made much more efficient use of bandwidth. The memory and cpu of both were used to match the rest of the hardware, so you can't just directly compare one spec and say it's better. Had PS2 had a modern gpu, it would not have had anywhere near the bandwidth. Likewise, the cpu may have been designed differently.
The Wii is a perfect example of this. That console is several more times less capable than the 360 or PS3. Mainstream users do not care. The value of functionality and features is greater to them than the tech specs on paper.
Even better example, the Wii is supposed to be 3x the power of gamecube, but it doesn't even match it in graphics!
AMD = ATI, but anyhow ATI (and I think even nvidia) are already producing graphics chips on an 80nm process.
