Memory size and speed, or RAM, can significantly impact performance, and depending on the application, could influence throughput more than anything else in your CAD workstation. Usually there's a sweet spot. To find it, start with the minimum recommendation for your primary software, then get a feel for how much more memory you'll get with incremental spending.
Performance versus Budget
To achieve solid performance within a reasonable budget, that sweet spot today is likely between 6 GB and 16 GB of DDR3 1333-MHz RAM. DDR3 is third generation, dual-data rate memory technology, with Intel's current platforms centered on 1333-MHz clock frequency — and it's really your best memory option these days.
Also pay attention to how many of your system's dual inline memory module (DIMM) slots are taken up by the system memory. This should be clear from the system specs and from the system configurator when purchasing a system online. For example, 4 GB might be specified as "1333 MHz, DDR3 SDRAM, ECC (4 DIMMs)," meaning that four slots are occupied (out of the total number of slots specified in the model's spec page or datasheet). Ideally, you'll want to leave some DIMM slots empty so you can give your system a mid-life memory upgrade if needed. Depending on the density you've chosen, leaving empty slots often involves no additional cost.
Error Correcting Code
And what of Error Correcting Code (ECC), an upgrade that typically allows single-bit memory errors to be detected and corrected? New Xeon processors offer integrated ECC, but with other processors it's an added expense. For most CAD applications, ECC is certainly valuable but not essential. If the added cost is modest and doesn't sacrifice performance — sometimes the DDR clock frequency must drop to accommodate ECC — go for it.