I would agree with the SSD. I've just put one in all 3 of my PC's (2 desktops and one laptop) and the difference is like night and day. Boot up times, even on my elderly Dell XPS M1530 laptop with a Core 2 Duo T8300 2.4ghz, is like 15 seconds to the log on screen, then after logging on, I can use a program or surf the web in another 4-5 seconds. Even faster on my quad core desktops ! If you are just using it for the OS and programs (i.e. not storage) then a 128gb SSD is perfectly big enough. On the PC I'm typing on, I have Windows 7 ultimate 64bit, PS CS5, Lightroom 4, Capture NX, Photomatix, Qimage, full office 2010 package as well as a load of other programs, and my SSD is currently showing 56gb used, 55 gb available (50%) - just to give you an idea.
64it OS is pretty much essential these days. You shouldn't really need more than 8gb RAM. Get a good Sata 3 Mobo and a Sata 3 7200 RPM large capacity HDD to go with the SSD for storage only and your set.
Processors is pretty much how much you want to pay. The core i5 2500K is very overclockable (I have one in a desktop overclocked to 3.3 ghz, with standard air cooling, running standard voltages, that always stays nice and cool, and gives a big bang for your buck). Might also be a good idea to look a Mobos with USB 3 support as well.
Unless you want to play games, the built in Intel HD or AMD Radeon graphics are perfectly upto the task of most photographers (PS, Lightroom, NX etc), without the need of a discreet graphics card. If you want more power in the graphics side, it's really a case of how deeps your pockets are, as the high end Nvidia and AMD chipset cards aren't cheap.
Extras like DVD Roms, Blue Rays drives, card readers etc, can all be retro fitted or swapped out at your leisure.
Hope this helps.
