Best graphic card for PCI Express 2.0 (For COD: Ghost)

Hello

I have a gamer PC from 2008 and I now want to upgrade it so that I can run Call of Duty: Ghost decently.

I started out with this:

Processor: Intel Core 2 Quad Q6600 2.4 Ghz Socket LGA775, 8MB
G-card: XFX Geforce 9800GTX 675 512MB GDDR3 PCI e. 2.0
RAM: Corsair TWIN2X 6400C5DHX DDR2, 4096MB
Powersupply: Corsair Powersupply 750W Black
Motherboard: ASUS P5N-D, nForce-750i SLI, 2xPCI-Ex (2.0)

Call of Duty Ghost requirements:

Minimum System Requirements

OS: Windows 7 64-Bit / Windows 8 64-Bit
CPU: Intel Core 2 Duo E8200 2.66 GHZ / AMD Phenom X3 8750 2.4 GHZ or better
RAM: 6 GB RAM
HDD: 40 GB HD space
Video: NVIDIA GeForce GTS 450 / ATI Radeon HD 5870 or better
Sound: DirectX Compatible Sound Card
DirectX: 11
Internet: Broadband Internet connection and Steam and Online Multiplayer



Recommended System Requirements

OS: Windows 7 64-Bit / Windows 8 64-Bit
CPU: Intel Core i5 – 680 @ 3.6GHz
RAM: 8 GB RAM
HDD : 40 GB HD space
Video: NVIDIA GeForce GTX 760 @ 4GB
Sound: DirectX compatible sound card
DirectX: DirectX 11
Internet: Broadband connection and service required for Multiplayer Connectivity. Internet connection required for activation.


I'm going to apologise in advance for my limited knowledge about all this. The time where I could keep myself updated on every new piece of hardware is sadly gone. That's why I'm here asking the guys who knows their stuff.

So, here's the deal:

At first I bought 4GB more RAM (Corsair TWIN2X PC6400), so I'm up on a total of 8GB. I think that should do.

But then I got a message that my graphics card wasn't good enought to run the game. Beeing reluctant to spent alot of money on this, I bought the ASUS nVidia GeForce GT 630 2GB. It didn't work out that well... I can run the game, but it lags a bit on low graphics.

SO, what type of graphics card can I buy when my motherboard is PCI Express 2.0? I don't need everything on high. I just want decent graphics (so I guess normal/mid) and most importantly that it runs smoothly(!). If I understand correctly, I'll be able to use PCI Ex 3.0 cards, but they won't perform to their maximum effect. I'd be nice to buy the PCI Ex 3.0 card that gives me everything I can get out of the PCI Ex 2.0 slot.

What I fear is that my processor isn't good enough to run the game smoothly even when I spend a decent amount of money on a graphics card. Is that something I need to worry about? Or can I buy a card with confidence that my PC will be able to run Ghost smoothly?

To be clear, this is what I have now: http://www.komplett.no/k/ki.aspx?sku=752358

TL;DR /To sum up:

What graphics card can I buy for my PCI-Ex 2.0 slot to run COD: Ghost with decent graphics without lag?
And is my processor good enough to run the game without lag?

Thanks in advance!