Buying a PC might end up being cheaper for things like business computers. Mass produced, low end, not good for playing games at all.
Though one for gaming? Building it will save you more by far. In fact, I think I saw a combo that would get you a mid-end gaming PC if you got a different graphics card with it, too, and it was only about ~$300 all together for the tower. Though I don't really suggest the PSU that comes with it, looks iffy, if not just plain bad, and the HDD was extremely small capacity wise (160GB).
But if you built one with that card and all (what I think is) reliable parts (and plenty of hard drive space), it would cost about $450, and it would be able to run Crysis on high pretty well at 1680x1050. Then there's another $100 for Windows 7, and then you could re-use your current monitor, speaker, keyboard, mouse, etc., unless you're using a laptop right now...
I do not suggest going with a bad PSU. I've had a PSU go out on me before. I never plan on buying from Dell again. >_>