Any possible way to RUN Bf3/ Cod MW:3

Your problem is not just graphics, it's your CPU. It doesn't matter what graphics card you buy, you still won't be able to play these games, even on low settings. A Pentium 4 is below the minimum system requirements for either game. All modern games require at least a dual core CPU. Even with a dual core CPU, a 280 Watt Power Supply isn't enough to power any decent video card, the best you could possibly get would be the Radeon HD 5570. Everything else, even the 5670 would draw too much power. You also need to upgrade your OS to Windows Vista/7 in order to play Battlefield 3.

Honestly, you would have better luck trying to play either game on the laptop you are going to buy. I'd say save the money, and put it towards getting a decent GPU on the laptop you're going to buy. Your only other option is either upgrade both your CPU (if that is even possible) and graphics on your desktop, or build a new desktop.

It will probably be cheaper to buy an Xbox 360 or PS3, or upgrade your laptop you're going to buy.

The HD5670 will get the job done for MW3 and for battlefield 3 on low settings.

but first to play battlefield 3 your going to need to upgrade:

Your OS to Windows 7
Your CPU to a Quad core
Your motherboard
Your PSU
Your memory
and then your graphics card

Only a moron will buy a console.$600 gives you a decent quad core CPU with a gts 450 and it will destroy the console visually

Considering the OP would need a new motherboard/psu/ram and new OS on top of the fact that they probably will have to pay someone to put it together for them.. yeah, no.

Not to mention the fact that a console costs MUCH less than $600

In this case, sir, you are the moron.

Agreed, this is basically what I said in my post.

Console or ugrading GPU on his laptop is the way to go.