Best possible GPU for my PC?

I second this. Extremely low power consumption, cheap price, around $80. after rebate during this season, and great price:power ratio while being able to go full throttle ( for as much as that's worth ) with a budget CPU makes this card a definite looker for someone in your situation.

Either that, or buy a better CPU and then buy another GPU.

Up to 150$

Well, considering he has an underpowered CPU, I don't think he's looking in the 7970 range, or planning on buying a Titan, so I'd say it's probably a pretty appropriate title.

"you need a CPU not a gpu my friend". Nice troll attempt dude. i3 equals performance of older quad core CPU's and most of the games still require a dual core, GT440 is low end and must be upgraded.

I think AGP haven't been used in computers for years now, CPU-Z says it's PCI x16

The recommended power is already plenty. If his current PSU can hold out, there's no need to spend more money on a new PSU if all that extra wattage isn't going to be consumed.

that is true only if he is overclocking the cpu and gpu otherwise you are good with a 500/550 taking into consideration that last gen intel cpu are very good in saving power compared to older gen

So wrong, so so wrong. You can run ALL GPUs with a good 480W PSU. (Like I said, single GPUs)

Wanna know why the manufacturers keep saying you need atleast 600W? Because most ppl are still buying cheap crappy ones.

A higher wattage psu won't bottleneck anything

The PSU just provides power to the various components. Having a 1000 Watt PSU won't bottleneck your other hardware at all....

However, if your PSU is some unknown brand and not 80+ certified, I would suggest an upgrade to a PSU from a known brand like Corsair or Cooler Master (and one that's at least 80+ bronze).

Well, then you are good to go :)
later on u can upgrade as you go, as for right now you will be served, just dont aim for AAA titles, next gen games are a bitch :P