Build your own computer. There are tons of guides on the internet. It's basically legos, you will not screw up your hardware. That power supply is much too weak, and that entire system isn't very upgrade-friendly as it is. You'd want a 500-600W+ PSU, a case with nice airflow (such as Antec Nine Hundred), and a nice motherboard that supports multi GPUs at 16x/8x or 16x/16x. You should also consider the fact that 2gb of faster memory is much preferable to 4gb of 800mhz memory, a mistake many people seem to make.
If you're not dead set on buying a prebuilt PC, I can help you choose compatible parts within a price range.
None of you people saying HD content (1080p) looks "marginally better" than DVD content (480p) have any idea what you're talking about. The visual quality is astounding, and for people with excellent speaker setups, the audio quality is astounding.
If you're watching movies on your friend's SDTV with built-in speakers, you're doing it wrong.