I bought this HP Compaq Presario CQ3495AN PC about a few years ago now, and was well aware of its low specs. I wanted to upgrade into something better, and willing to spend about 200 - 400 AUD, and you guys seems to be the best people around to ask.
IF your looking at maxing out your budget, get an amd r9 280x for yourself, new card, performs almost as well as a 770 and for $100 less, and yeah australia gets ripped of for their graphics cards. If your only willing to spend your lowest part of your budget, a gtx 650ti would be good or maybe at $300 you could get an r9 270x or a stock-ish gtx 760
Then it has an i5 650, so there would be a bottleneck if you went for something like a 280x, especially since OC'ing in a prebuilt case with no airflow would be a bad idea. At the same time you should get a new power supply (if the one it currently has is the 250W). A 2 gig 7790/ 260x or 650 Ti Boost should be below $200 aud as far as I'm aware.