Client machines implies secretaries and such. UDMA66 is good for this application. It is unlikely they will benefit from the extra throughput of the UDMA100. You are talking "Upgrade", this implies older machines. If the motherboards already copewith UDMA66 then great. Extremely unlikely the motherboards support UDMA100 so getting an advantage of the faster drive is unlikely. Also very likely is the existing motherboards only support UDMA33.
Combining the two factors, low demand applications and older motherboards I would stick with UDMA66 hard drives.
Of course if performance is an issue because maybe these systems are used by architects or rocket scientists or something then the landscape changes. I have to wonder about the wisdom of using adapter cards to get the capabilities of UDMA versus using a motherboard with this support included. Remembering the KISS principle anything added to a system represents more things that can (and seem to usually happen on Friday afternoon) go wrong. Another consideration, boosting the performance of the hard drive won't make any appreciable difference if the rest of the system is "slow." Upgrades are usually only a viable solution when client is looking to save money (this rules out UDMA100) or have run out of space.
Evaluate the usage of systems before deciding on hardware. Performance needs are dictated by the usage. Demanding applications such as CAD automatically means more, bigger, faster of everything. Word processing can get away with substantially less and the user wouldn't notice the difference in any way that qualifies as legitimate. Loading Word in 3 seconds versus 5 seconds doesn't qualify as an urgent matter. Search and Replace, moving from beginning to end, Block and Move, these operations are more likely to concern the typists and if they are slowed down here then the specifications for RAM are not adequate.
