Dollar per dollar, RAM gives you more performance than the same number of dollars spent on a faster disk subsystem. If your budget doesn't allow you to full populate the RAM in your server when you first purchase it, be sure you leave room for adding more RAM so if you later find you need it, you can easily expand it. Invariably, if you try to save a few dollars now by purchasing less expensive RAM chips, you will often find out that your server needs more RAM sooner than later. And when this time comes, instead of just plugging the new chips in, you will have to toss out the old chips and buy all new chips in order to get the total amount of RAM you need. In the end, this is much more expensive than buying the more expensive chips in the first place.
Parallel queries use much more RAM than non-parallel queries.
And as you know, Microsoft always underestimates the amount of RAM you need to run its software. Because of this, you may want to consider doubling these figures as a starting point to determining how much RAM your SQL Server should have. Instead, always choose a RAID array made up of two or more physical drives that offers fault tolerance. Each of these RAID levels offers different performance levels. Ideally, if your budget can support it, chose RAID Level 10, which offers both high-speed and fault tolerance.