> the two searches above yield almost 10,000 results.
If you are buying Pentium 4's, look for the later 6-series CPU's that are hyperthreading and can run 64-bit Windows. There are faster ones>the Pentium 630 is 3.0Ghz., there's 3.2, 3.4, 3.6 and a Pentium 4 670 is 3.8Ghz for example.
Pentium 4 systems with the higher speed CPUs and hyperthreading can be surprisingly fast (I have a 2004 Dell 8400 that I used for CAD) but, unless your budget is extremely restricted, I would suggest concentrating on dual core.
If you are buying dual core, avoid the Pentium D and buy Core2 Duo. Look for CPU's like the E8400 3.0GHz. A Dell Optiplex 780 with and E8400 and with the mid tower case would be a good choice. There were many slim desktops and these are difficult to fit modern video cards-some have special right angle brackets. Look for the bigger cases if you have the space. The AMD Athlon dual core CPU like the Athlon X2 64 are O.K. but I suggest staying with Core2 Duo because of the larger number that were sold.
Consult this list of CPU's to compare performance >
I think you will find that these old systems are quite inexpensive, an the faster ones are not necessarily more expensive. If you need them all working in a hurry, make certain that they are complete with Windows, graphics cards, and hard drives or they will require finding many parts and days of work and a new hard drive may cost as much as the entire system. The RAM for those systems of that time can be very fussy to find the correct model. I once added RAM to a Dell Precision 380 and I had to buy it twice to find the exact one needed. If there is no OS, but the system has a readable COA, you may be able to buy inexpensive (these are often on Ebay for $10.00 or so) "reinstallation disks" that will install the operating system using the original numbers.
If you visit the Dell "official factory refurb store" ( I think that' the name or similar) on Ebay, they seem to have systems that are ready to use at quite good prices and inexpensive shipping.