World's fastest in what? There are many "world's fastest" metrics for computers. Since we're talking personal computers, I'll assume that if it can fit in a tower case (ATX or EATX) and can run Winderz, it's a PC.
1. Fastest clock speed:
-> Get a Pentium D 965 or a Pentium 4 671 Cedar Mill and put that sucker on water, Peltier, or phase change cooling. Overclock to around 5 GHz as that is very fast and will have at least a minimal lifespan.
2. Fastest in single-threaded application benchmarks:
-> Get a Core 2 Duo X6800 and overclock on water to about 3.5 GHz.
3. Fastest in multi-threaded application benchmarks:
-> Put two 3.00 Woodcrest Xeons or Opteron 290SEs on an ATX or EATX motherboard. The choice of Intel or AMD will vary with the application- Optys are better at server-type applications and the Xeon is better at most other things. Overclock if you wish, but DP servers are not as friendly to overclock if the board even permits it.
4. Most FLOPS:
-> Two 3.00 Woodcrests should do the trick.
5. Best GPU performance:
-> Get a decent motherboard that supports two x16 PCIe slots in SLi. The NVIDIA NForce 570 SLi, 590 SLi and NF4 SLi x32 fit the bill. Put two GeForce 7950GX2 boards in the slots, giving you four GPUs. The CPU choice does not matter that much- something like an Athlon 64 3000+ should keep the GPUs fed, especially at high resolutions.
6. Best overall performance
-> Put a pair of 150GB Raptors in RAID 1 on a reasonably good motherboard. The choice of CPU does not matter too too much- a Conroe E6700 or an Athlon X2 5000+ would be enough, I think. Put a X1900XTX or a 7900GT in the graphics slot. Put 2GB of good RAM in the system and make sure to go through MSCONFIG.EXE and pare down the useless startup services from the Windows installation to speed responsiveness and bootup speed. Your boss will see that this is a very fast system and be quite happy with its performance in office apps. The hard drive speed and RAM size is really the kicker- if you need more speed, you can RAID 0 those HDDs (I would not recommend it) or pull out the big guns and drop a couple of 15K SCSI drives in there. XP really only likes 2, maybe 3GB RAM, so unless he is using Linux or something, more than 2GB RAM is not going to help.