Your choice of Random Access Memory is of particular importance this year. But let the buyer beware: Don't be dazzled by high speeds. The fastest memory available, RDRAM (Rambus DRAM), is one to avoid. Offered by a single supplier, its high price tag was one of the factors that slowed the adoption of early Pentium 4s, and nothing has really changed.
The relatively new Double Data Rate (DDR) SDRAM, however, will ultimately prove to be the sweet spot for your high-performance Pentium 4 and Athlon systems when motherboards and supporting chipsets become more widely available and therefore cheaper.
Even then, traditional SDRAM will continue to be the best buy for most workers' desktops--it will have enough data transfer headroom for all but the most rigorous 3D applications for quite a while, according to technology analysts. If most of your company's desktops run SDRAM, sticking with that memory flavor will save you future hassle because it will simplify inventory, upgrades and tech support.
When you do migrate toward DDR, the cheapest way to buy it will be wrapped inside a new system. This way, you can avoid the cost of audits, installation and worker downtime. But of course, you may need to add more memory to some systems to keep them at near-optimal levels in the interim.
Gartner recommends a minimum of 256MB for a midrange system, and 512MB wouldn't go to waste. Memory is like duct tape--there's hardly anything that a little more of it can't help.