it all boils down to hard drive speed, when you open up an application like lightroom or photoshop it has to read the hard drive and the more things its doing the slower it is which means the longer it takes for the app to open
I messed around a lot trying to get good performance vs costs and what I ended up doing was putting a SSD for the OS & also the apps on it, then putting the cache files from LR / photoshop on it and then for my pictures I put 2 disks in raid0 to get max speed.... and keep in mind all HD's are not the same !
in a perfect world having a SSD to boot + apps, then another for the cache files + another for the pics but that would be crazy expensive
when you import a photo into LR for example it copies the file to wherever you choose, it renders a copy based on your setting and by default its not 1:1 and puts it in the cache folder, then when you go into a 100% zoom of the photo it has to render it
for the copy its really a mute point the memory card is the slowest thing so it doesn't matter if you have a SSD or a slow HD its not going to impact it
when it does the import rendering on the default settings its reading the file from wherever you store it so the drive is the better, and then of course its writing the cached file to the cache location so again the faster the better
then for the 100% zoom(maybe by default when you enter develop also actually) it does a complete render so again the faster the HD is that its reading the better, and of course then it writes that render to the cached folder ... so this step whole is really where you want the disk IO
As an example I initially had a single HD with OS / apps / cache on it and then had an external disk with my photos, it was so slow it drove me crazy ! I swapped the HD for a SSD and LR / Photoshop opened fast but it was still fairly slow rendering and I figured out the external disk IO was the problem. Then I put in 2 x WD black HD's in raid 0 and put the photos on there and holy crap what a difference that made.
One of the most annoying things for me was when you were in library and you pulled the slider to the left so it displayed as many photos as possible and then you would scroll and it would show blank photos and then they would slowly appear that drove me nuts. Now it rebuilds them so much faster.
so if your looking to build a machine my recommendation for cost is to do what i did 1 x SSD for apps + cache + OS and then where your storing your photos get whatever you can afford, at a minimum get 1 disk that is fast like the western digital black but if you can swing extras and put them in raid 0 you will love it .... of course all of this is great BUT you need backups as raid 0 has no redundancy and SSD's are not the most reliable ! if your using MAC its simple just use time machine, on PC use crashplan its free and works well, I use both and back everything up daily to an external raid
one last note is on processor, definitely that helps during the rendering so the faster the better, get an i7 if you can
sorry its so long