The 2600k with HT (hyperthreading) is barely better than the 2500k (no HT) in today's gaming.....however it has a huge performance difference in mutlthreaded apps. If your engineering apps are mutlithreaded, then the 2600k is a no brainer.
Anything that benefits from HT, will also benefit from more, faster and lower CAS RAM. That being the case I'd look at RAM speeds as recommended by your app vendors and not reusing your old Kingston stuff.
According to nVidia, ya wud need a 650 watt PSU for twin 460's
XFX Core Edition is $60 after MIRhttp://www.slizone.com/object/slizone_build_psu.html
And yes, forget the sound an network cards