For the quoted speed difference, is that for all computer related stuff, or mainly for photo editing speed issues.
Like MissionMan said, HDD's play an important role for some things, but not so much for other stuff.
If your speed difference reference is (say) for gaming ... then obviously the graphics card will make a difference.
But for photo editing, the graphics card make little difference.
Where you can eke out some extra speed in raw/tiff editing is in having a fast HDD, a large cache(for the software to use) .. you already have more than enough RAM.
The HDD on your desktop .. what model is it.
While MM mentioned that a HDD can run at 80Mb/s, and I tend to favour the 100-120MB/s models!! ..
.. you should note that some of the cheaper(and greener) HDDs can really only run at 50MB/s or less. I have WD 'Green' model it it's speed simply sucks!(there's no other word for it other than crapola!)
It's top speed is about 35-40MB/s.
Place any raw files(or large tiff files) on it and and watch it slow down every raw software like molasses .. in winter! Unbearable stuff.
But the difference in operating speed when those files are served from the 120MB/s Seagate drive is incredible to watch.
I don't do gaming .. so can't help if that is an issue for 'ya.