Seeing as the xbox has, effectively, a custom Graphics chip, that could prove a little difficult to answer.
I'm under the impression that the xbox has a triple core above 3GHz, but I'll have to look up the graphics situation.
EDIT:
I google'd 'xbox 360 specs' and got this:
A very helpful site with all the xbox specs on
This is the graphics part specifically:
Custom ATI Graphics Processor 500 MHz
10 MB embedded DRAM
48-way parallel floating-point dynamically-scheduled shader pipelines
Unified shader architecture
Polygon Performance 500 million triangles per second
Pixel Fill Rate 16 gigasamples per second fillrate using 4X MSAA
Shader Performance 48 billion shader operations per second
Memory 512 MB GDDR3 RAM
700 MHz DDR
Unified memory architecture
In comparison (My HD 4890, roughly equivalent to a GTX275):
ATi Graphics Processor 900 MHz
Pixel Fill Rate 14.4 GPixels/s
Memory 1024 MB GDDR5 RAM (@1GHz)
Basically - the 360 is highly optimized to specific settings. Ever noticed how you can't change the MSAA or Filtering or anything else in-game? Or wondered why every game's graphics look almost identical? It's because a specific number of the various components (shader cores and what-have-you) can be made so as to keep costs down whilst getting acceptable performance. The lower levels of video-ram in the xbox are a prime example - it represents perfectly the limited resolutions that it's possible to play a 360 at. Non-HD, 480i/p, 720i/p or 1080i if I remember correctly. (1080i is actually a similar resolution to 720p. The limited power of the xbox is shown here because it cannot support 1080p (full HD) resolutions at acceptable frame rates).
In other words, most any new/post-2008 graphics card will be more powerful than an xbox-360, but if you tried to run an xbox 360 game to compare, the 360 would run it better because it is optimized better.
The two are barely comparable, as one is a completely custom chip and the neither can play the other's games without being hacked first. Which won't do any good for performance.
Hope this helps somewhat.
PS. In some new games (like Need For Speed: Shift, which is basically an xbox port) your CPU will limit frame rates, so this will affect whether your PC 'feels' better than an xbox. Oh, and that game is also game-pad orientated, so that will have an effect too.
