A gigabyte ds3 or asus p5b-deluxe are good, solid motherboards for the conroes. Not sure what psu you picked out but a 550w is overkill for that system. A froton 450w (prob a 400w is enough but a little room for a better gpu later is good) would be plenty, and use the saved money to get a better gpu like a 7600gt.
oh and 4gb of memory is way overkill and actually slows xp down. I would wait on the extra 2gb until you can get vista. And an e6600 is a sweet processor but do you really need that much power for 3d apps. A e6400 is probably good enough i think, since the 3d apps are more graphics heavy, right?
If you decide to save the money with the froton and e6400, id get a x7950gt and you should be able to play current games and run 3d apps with ease.
edit: don't get an ide hdd, you should get a sata 3gb/s hdd.
CPU: e6600 @ 3.2ghz
RAM: 2GB G.Skill HZ PC6400
Mobo: ASUS P5B Deluxe
GPU: Sapphire HD4870
Soundcard: Creative Labs X-Fi XtremeGamer Fatal1ty
HDDs: 250GB WD + 750GB WD + 2x1TB WD
Monitor: Samsung 226BW
PSU: OCZ GameXStream 600W
Case: Cooler Master Stacker
OS: XP Professional SP3
Mouse/keyboard: Logitech MX518/Saitek Eclipse II
Speakers: Logitech Z-2300