Hi Hardball.
Sounds like you're in "Oh, balls, I need to throw some money at a better GPU, in order to play CoD 'well'; but which card?" labyrinth too!!! And it is a labyrinth, believe me.
Welcome.
Firstly, as unknownraver says, it's a tad early to be able to say definitively, for example, which CoD will benefit more from: more memory or faster clock/memory speeds. At the moment it seems to like more memory, but who knows if that will change.
I'm going through the same card selection process too, because my card is struggling a little, but I'm going to hold fire to see how the game evolves and what it benefits most from in terms of GPU spec. To be honest, I play mostly offline, and I can't see that CoD has much to offer as yet.
However, enough about me!
Have a look at this site, which I thought was really useful:
http://www.videocardbenchmark.net/high_end_gpus.html
You can also select "Price Performance" at the top of the page, and this gives a very useful guide. In essence NVidia cards perform better but are more expensive per unit of performance.
My last 3 cards have been NVidia, so I'll probably stick with them, and also because the Radeon 6900 series have received a fairly cool reception so far.
You mentioned your PSU. Why are you restricted to 500W? Is that your current PSU rating? If the PSU's the rate limiting factor, maybe you should replace the PSU before the card? They don't have to be very expensive, but just ensure that you have sufficient amperage on the 12V rail to fuel the card and the rest of your PC.
I only know this, because my PC is crashing and rebooting itself whilst playing CoD, and I'm pretty sure it's because the game massively stresses out the GPU, and then my PSU can't cope (the newer NVidia cards require 40A on the 12V rail, and my PSU delivers less than that).
So, in summary, make sure that your PSU can support whatever card you get, because CoD will be maxing out that puppy! No point having a great GPU, if your system can't sustain it.
Hope this helps, and let's also hope that CoD justifies all the money we're throwing at this game.
R