CHAINTECH S1689 Socket 939 ULi M1689 ATX AMD Motherboard - 68 (cheapest s939 mobo)
amd athlon 64 3000+ - 146
SAPPHIRE 100120 Radeon X300SE HyperMemory 128MB DDR HyperMemory (32MB Onboard VRAM) 128-bit DDR PCI Express x16 Video Card - 48 (very crappy video card. more for video editing isnt it?)
262 dollars for a video card, mobo, and cpu. you still need a case - 20 dollars, dvd burner - 40 dollars. OS - 81 dollars. hd - 53 dollars
thats 456 dollars right there. no where near 300 dollras...
EDIT: and btw you really think you can build a bottom line computer for 300 dollars? why dont you check out www.newegg.com
look at the parts yourself and see how expensive they are...
OH YEAH I FORGOT THE RAM! lol the ram will be an additional 50 dollars for 512 which most people have 1GB these days
so thats 500 dollars for a whole computer