turneranthony,
That's a loaded question and there is no short response.
It depends on what you're looking for exactly. The newest games that are coming out in recent months and are DirectX11 based? Then you'll need a total rehaul of the system. Right now I suggest going with Intel's Sandybridge 2500 processor and a supporting motherboard. A P67 chipset motherboard seem to be the best option today to go with that processor. A Graphics card such as ATi 6870 which gives best bang for the buck in my opinion(anyone welcome to argue that).
4 gig of ram, in todays gaming standards is a minimum.
In reality if you looking to spend very little I'd say that your bottleneck must be the graphics card. A cheap upgrade to that should give you a decent performance boost.
For the ease of understanding how the ATi's graphics cards are numbered you should look at the first two numbers. First one being the generation(graphics API support, memory type or other HW updates) and the second being is the "series" of the generation which usually represents the power of the video card(clock speeds and number of processors) and often the driver type to download. Third number simply further breaks down the power but to a lesser degree. In other words an ATi 5850 is still more powerful than ATI 5770.
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