Help in choosing GPU + PSU or motherboard + CPU upgrade

[PcEthuaist]

Allright, I have a 400 dollar budget to upgrade my existing rusting pc. What I want my upgrade to do is improve my pc's multitasking and quality and fps in games (gaming is the most important thing). Right now my current build is this:

Pentium Dual-core @ 1.8 ghz
Galaxy GeForce 8400GS 512MB DDR2 PCI Express
2 gb ddr2 ram
250 watts psu
P5LP-LE Lancaster8 motherboard

it's a compaq by the name of SR5262NX

heres a link to specific details of my pc (you can laugh, I know its a prebuilt)

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01155058&tmp_task=prodinfoCategory&cc=us&dlc=en&lang=en&lc=en&product=3548670#N67

For the Gpu, I was thinking a card on the nvidia GTX 500 line, and maybe a small amount of the money for a 500 watt psu. For the Motherboard, and Cpu, the Motherboard supporting at least one pci-e 2.0 slot, atleast 16 gb of ram capacity, supporting intel core series and atleast one usb 3.0 port. (I know I have t upgrade my ram, but it'll probably be some cheap one gig for the moment). The CPU that I am aimign for is a Quad core i5 @ around 3 Ghz. Can you, the community help me determine if I should get a Gpu + psu or Mobo + CPu package, and then what products specifically?

Also, these are the games that I play:
Source Engine Games
Battlefield 2:Bad Company 2

Games that I wish my upgrade can play:
Battlefield 3
Crysis
Metro 2033
Far Cry 2

 

[genghiskron]

the key question, imo, is: do you plan to continue to upgrade the computer, or is this the upgrade thats going to last you for the next few years?

 

[PcEthuaist]

I will continue to upgrade other components When I get more money, and this might be on for a few years if the new components do not break. And if this upgrade that I am looking for seems to become obsolete in a very short time, can you recommend a better upgrade?