well for 300-400 you wont be able to get a gpu in there for sure, I don't think you can even get a half decent computer for that little
Poppyrooster. If he isn't buying an OS, is keen on buying package deals/sales, and cuts corners as far performance and in some arenas, quality, he should be able to sneak in there for around 400.
Now, of course, at this price range he'd be better suited just buying a prebuilt, and there's no way he's going to get performance parts for that price, but he could build a computer on that budget.