The PSU will not be sufficient. Get a 480W+ from Thermaltake or Antec, and make sure it has a 24-pin power connector. Some of the Newegg ones have it in their name or specs and some show it in the pictures.
For the motherboard, get a DFI nF4 Ultra, hands down. The only alternative I'd consider is the DFI nF4 SLI, but whether or not the possiblity of SLI is worth it to you/your uncle is up to you.
Get a GeForce 6800 Ultra or GT for the video card. You may want an aftermarket cooler, particularly the Arctic Cooling NV Silencer (I think it's 5 for GTs and Ultras). That will ensure it's quiet, and you should be able to overclock it significantly more. If he won't be gaming that much, make it a 6600GT.
Get any cheap keyboard that doesn't look like it sucks. If he likes keyboards with a nice shape and all the special buttons, get a Logitech elite.
For a mouse, the MX10000 is really nice, but if it seems to expensive for a mouse, get a cheaper (corded) Logitech, a Razor Viper, or any decent corded, optical mouse.
For RAM, just get 1GB (512MB x 2) PC3200 RAM from a good company. It should be CAS 2.5 if you want to save money. I think Mushkin and Corsair both have some pretty good deals on Newegg right now in that respect.
For your processor, either get a 3500 or 3800, depending on how much money you have left (whether you get a 6600GT or 6800* is the main deciding factor here). You could also just wait for dual-core. Some of the lower-end (e.g. X2 4200 for around $500) one's may be affordable, depending on your choices in other areas. It would be perfect for this system (gaming while leaving normal stuff running as most people do, even if I don't), especially if he does encode videos or try to have anything intensive run while still using the machine.
Get something like a Thermaltake K8 SilentBoost along with some Arctic Silver 5 should be quiet, but powerful aftermarket cooling.
For CD drives, get a dual-layer DVD burner and DVD/CD-RW combo drive. Lite-on and NEC are your best bets in either case.
As for a monitor, this ought to be good.
For hard drive, get something between 80GB and 250G, depending on his projected storage use. A 160GB drive would likely be the most economical, if he'll ever use that much. Go with SATA and for the larger cache drives if you get a size in which 16MB caches are offered. Go Seagate where possible for the five-year warranty. Western Digital or Maxtor otherwise, with Samsung as a possiblity.
For speakers, just get whatever Logitech or Creative offers. If it's 2.1 using onboard sound it's not going to make a huge difference.
For a floppy drive... All the cheap ones are $8.50, but see if any of them have free shipping or cheaper shipping than others. I know you like phase change cooling for your floppy, but just stick with no cooling for this. I can't believe I just typed four sentences regarding floppy drives