Time to upgrade! (Upgrade advice)

While I'm not sure if this is the actual section to post this, I was simply wondering if it was possible to slowly upgrade my system. Its a stock HP Pavilion p6360a.

Approximate Purchase Date: A few days before Christmas if possible.

Budget Range: $400 au, mostly on graphics card and case and most likely power supply.

System Usage from Most to Least Important: Gaming, general web browsing.

Parts Not Required: Well, from my testing, the graphics card is the bottleneck, and therefore needs upgrading the most, hence the CPU, motherboard, RAM, screen, keyboard and everything else that doesn't need upgrading to fit a new graphics card.

Preferred Website(s) for Parts: pccasegear.com

Country: Australia

Parts Preferences: No preference, compatibility and effectiveness are the only restraints

Overclocking: Maybe, though I doubt it.

SLI or Crossfire: No.

Monitor Resolution:1920x1080
Additional Comments: I would really like to know if its possible, because I'm pretty sure the CPU and motherboard are on an outdated chipset and therefore simply swapping the graphics card is fraught with problems and I would likely be better getting an entire new pc. However, I lack the funding, therefore any way I can moderately cheaply upgrade the graphics card, case and power supply, and then the rest later would be amazing.

Full Specs, or as much as I can find in one place.

http://bizsupport1.austin.hp.com/b [...] ntver=true

Thank you