memory has got incredibly cheap, about 83 to 108 euros here for 16GB or 126 to 155 euros for 24GB.
so even if you want to go cheap (i.e. a 500 euros PC) you would have 16GB memory as a baseline.
if no GPGPU (i.e. only CUDA worth mentioning) then you're only displaying 2D and video, an IGP is enough.
for a minimal computer I'm figuring out an am3+ 880G motherboard, phenom II X4 955BE, 16GB PC10600, two 7200 rpm hard drives (one as the "source" and one as "target" to spread I/O between them?), an Antec 380W PSU.
very cheap and we may build up from there
. or leave it around that maybe if you only want an usable rig, I guess it's more than usable, it stomps every high end laptop.
maybe have a raid 0, or even a raid 10 to store your source materials.
2600K is the obvious powerhouse. buying a used 1366 socket setup and putting 24GB would be a fun option (or getting it new with a slower CPU as the trade off)
graphics card for the GPGPU needs not be a huge gas-guzzling one so you may look at a GTS 450 with 1GB gddr5, or a GTX 550.
beware of windows 7 premium having an artificial 16GB limit