What to upgrade on a limited budget?

[Patriot17]

I'm looking to tweak the performance of my current rig but have fallen behind what is good and what is bad in the current markets. I currently run:

Dual core E6850 3GHz
Gigabyte P35C-DS3R Mobo
4 GHz Ram (think its 1066MHz)
8800 GTX - Dxdiag reckons its got 2Gb of RAM but I think it lies! More than likely the 1Gb version
Using a 24" Monitor in 1900x1280
Vista Home Premium
Other bits are pretty standard and fine, more than enough power on the PSU

Which of those items is in most need of an upgrade on the gaming front? I've noticed things start to chug in places and planning to be playing a large amount of Co-op Operation Flashpoint 2 when it comes out. I don't want to be the one causing issues.

Budget wise i've got about £300 pounds max, more than likely it would be £150 this month and £150 next month.

So whats the best "bang for buck" upgrade please?

 

[Patriot17]

Thanks for the advice,

I notice you have linked to a Nvidia card, is that because they are better performance for cost at the moment? I ask because what seems like performance wise to be similar card between Nvidia and ATI the ATI card is generally cheaper.

I was thinking the Quad core would probably be a good idea, is it worth trying to reach the next step up in terms of clock speed or is the performance gain (bear in mind that this computer is soley for gaming) not worth the extra few pounds?