Quad core is seriously nice, be it either AMD or Intel. Having the fastest costs the mostest though! So unless you are rich or commercially engaged, a slightly back from the bleeding edge will save you bucks and still delivery bang for those that you do spend.
I recommend at least two physical drives as well. If you don't raid you can still cross backup all the critical items on each so a drive crash will not stop you for long.
Not sure what you are using in a monitor but get a video card the can at least push 1600x1200 in 32 bit at greater than 80 hz refresh. Even though it's older my Nvidia FX7900GTX is a rock solid performer.
I have always been partial to ASUS mobo's, fewer returns than any other I have seen.

Pass no temptation lightly by, for one never knows when it may pass again!