This is a good question because getting a read on memory issues for performance monitoring is difficult.
First off, when looking at
Page Faults/sec keep in mind that this includes soft faults, hard faults and file cache faults. For the most part, you can ignore soft faults (i.e. paging between memory locations) and cache faults (reading files in to memory) as they have limited performance impact in most situations.
The real counter for memory shortages will be hard faults which can be found under
Memory: Page Reads/sec. Hard faults mean process execution is interrupted so memory can be read from disk (usually it means hitting the page file). I would consider any sustained number of hard faults to be indicative of a memory shortage.
As you go further down the rabbit hole, you can also compare disk queue lengths to hard faults to see if the disk reads are further affecting disk performance. To get a picture here, look at
Physical Disk: Avg. Disk Queue Length. If this number is greater than the number of spindles in your array, you have a problem. However, if this number only spikes during hard page faults, you have a problem with memory capacity and not disk performance.