I was just wondering what PC GPU(s) are equal power/performance to the xbox 360's GPU ?

ATi X1900 is about on the same level.

While in some cases the Xbox360 GPU may perform similarly to the X1900 series it is a very different chip; it has 48 unified shaders, integrated eDRAM and tessellation support. It's not necessarily that powerful of a GPU, but it's quite flexible in the hands of a developer who only has to optimize for a single hardware configuration.

Basically if they where going to port a 360 game over to PC then a X1900 would more or less (probably more on the former side) play it at the same speed as the 360 does, if used on the same Resolution/Quality scale of course. To be honest a X800 could probably keep up with the 360 GPU on a like for like quality basis.

Just my personal experience with Oblivion and a few other games I have played on the 360 and later on the PC.

But in technical terms I agree the 360 GPU is better but then technically technical terms mean jack when they are both producing the same picture at the end of the screen and the less technically advanced one does it faster.

It means that xbox360 devs will be able to stretch the lifetime of the console, future games for that console will still look better than games released today. That is of advantage to the console owner, but not for the decisionmaking (to get it for which platform) regarding a specific game.

Then again that all depends on how good the developer is.