Need help building a new computer

The Sonata III ain't bad for the price, considering it comes with a solid 500 watt power supply. The Gigabyte EP45-DS3L and Radeon 3850 are obvious selections for motherboard and video card, respectively, and any DDR2 800 RAM should be sufficient; the performance boost from buying fancy low-latency or high speed RAM is more than matched by spending the extra money it costs on a faster processor or video card. Just don't waste money buying 8GB of RAM unless you have a 64 bit operating system and are willing to deal with its annoyances.

Are you at all interested in overclocking? All of the Core 2 family processors will overclock like mad, and as such you could get tremendous performance out of even a lowly E5200 with a stock cooler, not that that processor is a slouch even at stock speeds.

Finally, if your hard drives are more than 2-3 years old, it might be prudent to buy a new one to use as primary storage, and use the old drives as backup storage.

Jam it back in, in the dark.