Depending on what your brother wants to do with this system, I don't think you made as big a mistake buying the P4 as the other posters do. You probably could have gotten something for a little less money that would have performed equally well, but that won't blow your budget. $90 for a CPU is pretty cheap. You could only realistically expect to spend $20-30 less, if we're talking about buying stuff new. So don't sweat it.
I generally recommend buying from newegg.com. They have a large selection, near the best prices, excellent service, and their website actually shows pictures of what you're buying. I find that extremely handy since I'm picky about stuff like whether motherboards have fans on the northbridge, etc. They also have a nice search facility for each "type" of hardware (motherboard, video card, cpu, etc.). Unfortunately, socket-478 Prescott is such old hardware that they don't have a very good selection. One thing you need to clarify is whether your brother wants to play 3D games on this machine. If he doesn't, then you can save money by getting a motherboard with integrated graphics. If he does, then integrated video is so poor that you'd be better off buying a separate card. You're not going to get a great card with that budget, but you can certainly get something that will outperform any integrated solution found on a socket478 board.
I'll assume for now that he wants to leave open the option of gaming. This mobo might fit the bill:
Supports SATA too, which isn't always the case for motherboards this old.
You're going to spend about $55-65 on a 80GB hard drive. Within that range, take your pick. Hitachi Deskstar 7K80, Western Digital WD800JD, Maxtor DiamondMax 10 6L080M0, Seagate Barracuda 7200.9. All of these have SATA models. Some support the newer 3.0G/s standard, but that won't matter since your motherboard will only support SATA150.
Here's the WD, for $55:
Get DDR400. I'd recommend 512M as an absolute minimum. A full gig is preferable, but on your budget I don't think you'll be able to afford it. I tend to avoid off-brand memory, and stick with companies like Crucial, Kingston, Corsair. Don't bother spending extra money on low-latency RAM- tweaking your system that way is pretty low on the "bang for your buck" scale.
This should work:
You still need a video card. Here's what you've spent so far:
That leaves approx. $70 for a video card. If you could spend a tad more, say in the $100-110 range, you could pick up a ATI X700 or Geforce 6600. Limiting ourselves to $70, though, you're looking at an ATI 9550/9600/9600Pro or a Geforce FX5500.
Here's a FX500 w/ 256M for $60 ($45 after rebate):
Here's a 9550 w/ 256M for $60:
There are also 9550's and 9600's available with only a heatsink (no fan), if you're interested in cutting down on system noise.