Aiming for a high end games machine is a good way to go about building a very good all-around machine. Computer gaming is arguably the most intensive function a PC can perform.
I'll echo Dave and say to go AMD rather than Pentium. Regardless of what Intel would have you believe, the AMD will outperform its Pentium "equivalent" 9 times out of 10. Do a Google for some comparitive benchmarks. There are tons of these about, and should be nice in your report. I'd recommend a 3gb+ Winchester. It should come with a suitable fan and heatsink, but you can get all exotic if you want to. Unless you are planning to overclock the chip ( why?! ), extra coolant shouldnt be required. Doing some research about exactly why I put speech marks around "equivalent" might give you some info for that report too
With a winchester chip you will need a socket 939 motherboard. The 939 refers to the amount of pins on the underside of the chip. Asus boards seem to be dropping in quality from what Ive heard from friends in the trade, but MSI are just getting better and better. Great prices too. Go for Serial ATA over IDE, a firewire input if your client will have a digicam, plenty of USB ports, and a RAID controller would be nice for ..
A nice big 400gb RAID Array
. This is when you run two drives in such a way as to make them believe they are actually one. The lowered seek time results in faster data access, and therefore a more efficient system. 2x 200gb 7200rpm 8mb cache Hard Drives from Western Digital wont set you back that much in the scheme of things. If you dont like the price, reduce the sizes accordingly. Another idea might be to splash out on a very fast smaller drive like a 35gb or 70gb 10000rpm Raptor to install your operating system and by extension programs on, and then buy a less expensive but bigger slave drive to store your files on.
How important is sound quality in your build? You can save money by using the on-board sound features most Mother Boards now come with, rather than buying a seperate sound card.
I dont know how important graphics will be, but I'd advise ATI cards over NVIDIA. The new PCIE interface is something to consider if you decide to splash out. Though the technology is a little too new to be practical at the moment, it could pay dividends in the future.
Buying a reasonably expensive, branded, "Matched Pair" of 512mb PC3200 DDR Ram Sticks and running them in "Dual Channel" mode will get the most out of them. 2 good brands are "Kingston"and "Crucial" Aside from the actual CPU, your RAM is your biggest friend. Dont bother with more than 1gb. Waste of money. I believe the maximum supported by Windows XP ( Im assuming this is the operating system you will use ) is 2gb, but you would need to be running a truely ridiculous amount of programs to need more than 1gb.
Buy a well made system case. Avoid the flashy-but-cheap boxes with free flashy lights and see-through system fans. It should have an air intake and a fan or housing for one at the front ( mine is concealed beneath the bottom of the front panel, and some sensible means of air output at the at the rear. 2 system fans should really be your limit. Anymore than that and your PC will sound like a jet is taking off inside it when you hit the power button. If you actually need more than 2 fans, its likely you bought a bad case.
You should then put/give/throw away the power supply. Its unlikely you will buy a case without one, as they almost always included. These are generally low wattage, noisy, and so badly made that the dust that invariably travels into them has no way to exit and clogs up the supply. Then go and buy youself a brand spanking 400 or 450w model with a nice quiet fan.
DVD-/+RW's are ten a penny now. Pioneer and Lite-on are both great brands.
Unfortunately, I cant really give you any good sites to check out for actual purchases. You might confuse your prof if your prices are in ££££'s rather than $$$$'s
I think thats about it