Help, poor performance in games!

Okay, so I just got an okay rig that was pretty much built after maximum pc's kickass 500 dollar pc..

Anyway, this is what I have.

CPU: E5200 OC'd at 3.0 ghz on stock cooling, using AS7
MOBO: EVGA 730i
RAM: 4gb OCZ BLADE 800mhz
GPU: Sapphire 4870 512mb
Hard Drive: 500gb Western Digital Caviar Black
CASE: NZXT BETA
PSU: 600w OCZ MODXSTREAM
OS: Windows XP SP3

So anyway, here's the problem.. I play games and then the gameplay is pretty sluggish, like in the Sims 3, I get around 20fps without any AA!

And here's the worse part! I play GTA4 on Medium Settings, and once i step outside I get 15fps!

It should be something like this..

http://www.youtube.com/watch?v=DvPt26stHRQ

But I'm running my games at 1440x900! Help please? I downloaded some drivers off ati's website..

I really think that GTA4 is simply a horrible port for the pc. I've heard they recommend a quad core. Surprisingly sims 3 is pretty hard on my system. it plays it well but my evga gtx 275 is practically baking after playing for a few minutes.
The only real problem i could see would maybe be a bad driver. and you might want to attempt to overclock your gpu. I really think overclocking a gpu is much easier then a cpu and on the 4870 and 4890's you can get a pretty good boost like 12-15%

I suggest you check your drivers this is a common problem but it will dramatically affect your pc's performance there are two things you can do, you can download drivermax for free which will check all of your drivers for you however it doesn't always have an updated list of drivers in this case i suggest going to the amd website and checking what the latest driver is then check what version you have and update if necessary.

Idle temperatures have little meaning, what is the load temperatures of the CPU/GPU?
Try another game, both those you have mentioned are very hard on the CPU, try a shooter instead.
Are you playing in a window? The HD4870 will not switch to full 3D mode if you play in a window.

I would agree that using a monitoring program to see clock speeds (CPU and GPU), usage (CPU) and temperatures (CPU, GPU, MB) while gaming would give evidence of throttling/overheating.