first of all, don't buy a pre-built machine (i.e anything you get from dell, harvey norman, good guys etc..). You should ring a normal computer shop and get a quote for a custom built computer (i.e: u research and select what parts to put in, and the people at the computer shops build it for u). For example, try arc computers. The main things to consider is cpu, graphics card, hard drive, motherboard, case (mainli for how well cooled it is, like airflow and fans). In the end, what your computer can do is basically dependant on cpu and graphics card, so have a budget of $500 for the two. For example, the nvidia gt260 is a good graphics card and the intel e8400 is a good cpu. Hard drive, go for 1tb, and that should be round $100-150. A good motherboard is around $150-200, and a case + power supply would be around $150-250 (for a good gaming case with fans included). A 22" monitor should set you back $200-300, and a set of logitec x530 speakers should be under $100.
in the end, you could make a computer that would basically do anything you want, and run basically every game (itll be like 5x better than ps3 and xbox combined when coming to games).
The above works only for desktops btw.