All prices estimates from , Newegg , which is in my opinion the best place to buy computer parts.
CPU Athlon 64 3000+ Retail (includes HSF) $165
Motherboard ABIT “KV8 Pro” K8T800 Pro $99 - note this board includes decent onboard sound, if it doesn’t work for you then look at other soundcards.
Video Card SAPPHIRE ATI RADEON 9800 PRO $197
RAM Corsair Value Select, PC3200, two 512MB sticks $157
Hard Drive Western Digital 160GB 7200RPM SATA - $100
Optical Drive Lite-On 12X DVD+/-RW Drive $64
Power Supply Thermaltake W0014 Silent Purepower 480W $55
Operating SystemMicrosoft Windows XP Home Edition $87
Caseinkworld Black/Silver ATX Mid Tower Case -$25.50 - Note cases are more personal items, I would go out to a store and look at them before buying; this is more of a place holder, though the pics of the case look good.
Floppy Drive Any will do, $10
Total Price - $934.
This will give you a very powerful machine, capable of playing any game out there right now quite well, and should handle all other tasks out there very nicely. Hopefully you can pick out a monitor/keyboard/mouse/misc extras for the $270-$370 you have left over; these items are best tested in person.
This machine includes a power supply from a good brand; you never want to be cheap out on the power supply, it is what it feeding electricity to everything else in your computer. Whatever you do sure you get a power supply made by Sparkle, Antec, Enermax, Thermaltake, or if you have a lot of cash, PC Power & Cooling. A 350 Watt power suppy from a good brand is much better than a generic 450 or 500 watt power supply.
Note that this computer has a faster processor, much faster video card, includes RAM, a bigger Hard Drive, a better optical drive, but has a worse sound card than than the system sleestak speced out.