Ah, different questions, so here goes.
Yes, if you have a VERY large amount of RAM (that is, more than you ever use with typical applications running), the OS should not ever use the Virtual RAM on disk. It should only use it when needed. Now, I am not absolutely sure that it NEVER would look at that disk file - the OS might do a minor check on it rarely and use very little time.
Now, having that much RAM still does NOT remove your HDD speed from consideration. Most normal operations need to access the HDD for several reasons. When you start an application, it must load from the HDD. When that app calls for additional features that are contained in auxiliary files like .dll's, they must be loaded. When the app opens an existing file, it reads it from the HDD. Any write operation accesses the HDD. Most apps have some limit on the size of RAM space they will use for handling a file. If the are working on a MUCH larger file, they may do so by accessing (that is, loading and working on) only a part of the file and later writing the updated info back to HDD before proceeding to loading another portion. Many apps include a timed automatic update feature - at pre-set intervals it writes the current version of the data file it is using in RAM back to the HDD for safe keeping in case of sudden failures.
So, lots of operations actually access your HDD on a regular basis, and hence the drive's speed is a factor in overall performance. However, many of the things you do like writing and editing a document are limited almost entirely by your own speed, and the HDD speed become inconsequential in those cases.
RAM is faster than and HDD, so in the old days it became common to use software utilities to create a disk drive buffer in real on-board RAM. The idea was to use this buffer between the HDD and the system RAM to speed up access to disk files. But for more than a decade (probably more like two) that function is done at the HDD's own PC board. It contains 8, 16, 32 or 64 MB or its own RAM plus a dedicated cache controller that does the job really well, and nobody devotes any mobo RAM or CPU time to doing it outside the HDD any more. This is all rather transparent, so you and your OS simply think of accessing the HDD, and never think of the HDD's cache RAM that is helping optimize performance.
The speed of the HDD is important in some operations - just how important depends on the operation. Gamers have found in the past that high-rpm drives with fast head access were desirable for them because they wanted minimum time to find the start of a file, because their operations used frequent access to many small files located semi-randomly on the HDD unit. Relational database apps used for lots of queries may also benefit from rapid head seek and disk rpm speeds. In some of these operations, larger on-board cache RAM may help because of the way it is managed (4-way splits). On the other hand, processing of very long files of sequential data is not affected much by head seek speed, but still is impacted by rpm's, and is a bit less sensitive to cache size. There are lots of other factors, too. To appreciate them, try looking closely at some of Tom's detailed HDD performance tests and see which types of operations are more impacted by which HDD parameters.
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