8bitgamer8.
It would take some research, but I think the most advanced CPU technology in 1998-2000 would be a Pentium III and I believe the fastest were 1.3 or 1.4GHz. It's possible that the IBM Power4, the first dual core CPU was available in 2000, but was certainly around in 2001 and I think that was a RISC CPU for servers- may not run Windows.
I still have a 1999 system, and one that has some of the better technology from that time. This is Dell Dimension XPS T700r with a Pentium III at 750MHz:
Dell Dimension XPS t700r, 1999
CPU: Pentium III > 750MHZ (Slot 1>)
Memory Size: 768 MB PC100 (768MB max.)
Hard Drives:1 > 30 GB Drive C: operating system and programs
. . . . . . . . 2 > 80 GB 2- partitions for data
HD controller: Ultra 66 PCI (SCSI) card (2 channels)
Removable Storage 1> 1.44-MB Diskette Drive
. . . . . . . . . . . . . . . . 2> iomega 100 backup drive
Video Card : AGP ATI Radeon 128MB
CD-RW : Memorex 52X CD-R, CD-RW, DVD-R Drive Ethernet network card :
Dynex 10/100 Ethernet/Internet
Fax Modem : 56K US Robotics
Soundcard: Audiowerk 2 PCI
1 ISA Slot and 4 PCI Slots
2 USB Ports
PS/2 Keyboard Port
PS/2 Mouse Port
One Printer Port (25-Pin Parallel, EPP/ECP)
One 15-Pin SVGA port
This was beautifully made and I think this system cost a total of about $2,500- the Audiowerk 2 soundcard was $250, and the SCSI controller which was twice the ordinary speed- 66MB/s instead of 33- was expensive- $200?- which shows how times have changed. There are people that still use the Audiowerk 2- even writing Windows XP and Linux drivers- it's very good. I have an NEC 3V 15" monitor from that time- about $500!
I think the successor was the Dimension l866r. ( "L"866r) and those could be from 866MHz to something like 950 or 1.1GHz.
Another vintage system I kept is a Dell Dimension 8400 from 2004 which has the first 64-bit, hyperthreading "Prescott" CPU. Only a few months earlier, the 8400 would have come with a 32-bit 570 CPU that was 3.8GHz and I would say that that might be a good CPU choice even though it's a bit after the period you mentioned.
http://ark.intel.com/products/27475/Intel-Pentium-4-Processor-570J-supporting-HT-Technology-1M-Cache-3_80-GHz-800-MHz-FSB
If the games you have in mind will run on a 32-bit system- perhaps with the hyperthreading disabled-, having a PCIe graphics slot, SATA drives, 4GB maximum RAM, and USB, I think something like that would be a much better prospect for vintage gaming, especially the PCIe graphics. A Pentium 570 at 3.8GHz is a $10.00 item and Dimension 8400's are around fro $100 and less. As so many application are single-threaded, I can run (on the 64-bit 630 3.0GHz) AutoCad 2007, Sketchup 8 and Adobe CS4 on a Quadro FX 580, and it actually has plenty of speed. I used that system for CAD and graphics until 2010.
Ah history!
BambiBoom
HP z420 (2014) > Xeon E5-1620 quad core @ 3.6 / 3.8GHz > 24GB ECC 1600 RAM > Quadro 4000 (2GB)> Samsung 840 SSD 250GB /Western Digital Black WD1003FZEX 1TB> M-Audio 192 sound card > AE3000 USB WiFi > HP 2711X, 27" 1920 X 1080 > Windows 7 Ultimate 64 >[Passmark system rating = 3923, 2D= 839 / 3D=2048]
Dell Precision T5400 (2008) > 2X Xeon X5460 quad core @3.16GHz > 16GB ECC 667> Quadro FX 4800 (1.5GB) > WD RE4 500GB / Seagate Barracuda 500GB > M-Audio 2496 Sound Card / Linksys 600N WiFi > Windows 7 Ultimate 64-bit >[Passmark system rating = 1859, CPU = 8528 / 2D= 512 / 3D=1097]
2D, 3D CAD, Image Processing, Rendering, Text > Architecture, industrial design, graphic design, written projects