$300 is doable. you can get a $189 e8400 (theres a few places online, though newegg isnt one of them)
newegg is $219.99 free shipping
http://www.newegg.com/Product/...E16819115037&Tpk=e8400
this gigabye g31 board is actualy fairly well recommended as the best g31
http://www.newegg.com/Product/...x?Item=N82E16813128078
its $67 + $6
there is also an ecs g31 for $55 after mail in rebate of $20 and it also has free shipping (you might be better of getting the gigabye though, i guess if you dont o/c it really doesnt matter)
wintec ddr2-800 2gb is $38.99 + $5 shipping (i actually have this memory and its fine and works great)
http://www.newegg.com/Product/...x?Item=N82E16820161172
memory has gone up slightly the last couple weeks... but still its definitely doable. newegg had some ddr2-667 for $3 less too.
so thats $39+5+67+6 =$114
plus the $189 e8400 and whatever shipping is for that. its not quite $300, but its barely over like $303. + taxes and whatever shipping is on the e8400 (or if he has a microcenter local he can get it there for $189.99)
i guess if he went all through newegg, got ddr2-667 and the ecs after rebate, and the newegg e8400 its $220+$55 after rebate, + $35.99 + 5 ddr2-667....
so thats $314 if he does it all through newegg at the absolute minimum with a rebate.