Consider building your computer: The computer in my specs was built for a total of $297 (never topped $313.) (Including replacing the DVD Burner and the Motherboard.) Original cost was somewhere around $186. The cost of a stock Lenovo computer (forgot the model but saw it at the local computer store yesterday) with the single-core AMD Sempron 2100+, the ATI Radeon HD 5450, 160 GB hard drive, AMD 760G chipset, is $500.
This computer would be acceptable for normal office use-the Pentium 4 was discontinued in 2008. It is single-core. You would not be able to upgrade to any of the new Intel Socket 1155 processors e.g. i3, i5, i7, and I think even the new Pentium, all support socket 1155.) The old core 2 duos run laps around this processor too. The simply horrible onboard Intel graphics will not work for anything other than, well, flash games at high resolutions. If you are looking to play, even, say, some of the latest games at, low resolutions, low details and with acceptable FPS, you need an upgrade.