basically, get a decent current processor, have separate drives for OS/program and Photoshop scratch, and stuff as much RAM as you can afford into the system...
for graphics card, anything that supports OpenGL 3 should be good, which is probably every current available card...
while multiple cores are not really useful in day to day stuff, some filters do use them, and Adobe Camera RAW seems to be able to use all the cores to process RAW files, with each core handling a RAW file in parallel; useful if you batch process multiple RAW files... get a quad core processor just to be safe if it doesn't blow your budget, but RAM should take priority in your budget...
SSDs for program drive will allow you to boot up your comp faster and load Photoshop faster... that's it... how useful that is depends on your preference and your budget... I do run my OS and programs from a OCZ Vertex, and Photoshop loads up in like <~3s even from a cold start...
the scratch drive should be on a separate drive to your OS/program drive... separate physical drive, not separate partition... having the scratch on a separate partition but on the same physical drive as your OS/program drive will not help...
you don't have to have a separate, isolated drive that you use solely for scratch... you do not have to set the space limit for Photoshop scratch, Photoshop will just have a temporary file written there as and when Photoshop loads up... you can store other stuff on this drive, but if you save your image files to the same drive as your scratch drive, saving will be slower (even if the files are saved to a different partition of the same physical drive as your scratch drive)...
WD Velociraptors are good, whether individual drives or RAID 0... nobody will stop you if you go for SSD RAID 0 though :bsmilie:... or if you have so much RAM that Photoshop is unlikely to use everything up, then you can do like Rashkae suggested and set some RAM up as a RAMdisk for ultimate scratch drive performance... but it's best not to do this unless your files in a typical Photoshop situation are running at >~90% efficiency or you'll just be taking usable RAM from Photoshop...
bottom line: more RAM helps...