System not running at full capacity

I have a system that seems to be lagging behind.

Everything outside of gaming is very quick and easy. But in most games I don't seem to be getting the FPS that I should.

I was thinking of getting a new case for the computer (as I only have the default case the computer came with, and have added a larger graphics card which may also be absorbing a lot of the power supply) with more fans and exhausts to keep everything inside cool. I was also thinking of upgrading the power supply. But I'm not too good with computers and so I thought I should ask.

Another thing is I may have a bodgy graphics card. I have a GeForce 9600GT by some random company called Zotac. The box said the card was 1GB of ram, but I later found that there was a sticker covering the bit on the box were it said 512mb. Anyone else think that the card is a sham? Or can they be modded with more ram?

Thnx.

Sorry for the late reply.

Processor: AMD Athlon x2 Duel Core Processor 5600+ 2.80 GHz
OS: Windows Vista (32-bit)
Memory: 4GB RAM
Monitor Dell E228WFP
Resolution: 1024x768

Now, I was unsure on how to find the exact specs of my PSU and motherboard but I'll give you as much as I know.

PSU: Inside the case on the side of the PSU was a sticker which had:

AC Input (50-60 Hz): 100-120V~/9A
200-240V~/4.5A

The motherboard had Dell written all over it, but had one small chip where it had the nVidea logo on it. Couldn't find any model numbers or anything, sorry.

Please help

Mainly World of Warcraft.

I have friends with computers that shouldn't be any where near as good as mine getting better FPS.

I understand that I can change the settings in game to give me better FPS, but then that would just be a waste for having everything I've got in my computer in the first place.

The frames I usually get is around 30-40 in the main city with lots of people around. But it can sometimes drop to 10 and even lower in other places. I was hoping I could fix the problem to make the FPS smooth wherever I go.

So I should get a quad core right?