Well, later this year Windows Vista is coming out, and with that comes a lot of new technologies. Such would be the introduction of DirectX 10 and Shader Model 4.0. There's all kinds of new things coming out this year, this is why I am suggesting to build your PC later on. There is NO point in spending money on a computer right now. Windows Vista will replace Windows XP in roughly 6 months, with that comes DirectX 10, which will make every video card currently out obsolete, plus, video games will use new Engines, and to top it all off, Physics Processing Units are coming out. This will make graphics so unbelievably good you won't even know it!
So, save up your money, don't buy anythign just yet. In six months there will be so many new things to choose from. Plus, nothing will be to horribly over-priced either. So your budget of $1,500 will still get you fairly far.