I just upgraded my formerly AMD system, and after a lot of web research picked something similar to your proposed system. I chose the little faster Core 2 Duo E6600 and went for the 2GB of memory up front with a “matched 2x1Gb set”, but your E6400 and 1GB should be fine.
I also got a 7600 GT, I don’t think it was needed for vegas, but I liked that it had no fan. I’ve gotten to not like fans, especially tiny ones because they’re both whiney/noisy/loud and in my experience unreliable (and often not replaceable, esp on video cards). So got the Gigabyte version of 7600 GT with no fan. Likewise got the Gigabyte 765P-DQ6 motherboard that has no fan. Both have big heatsinks and lots of heatpipe plumbing.
A real nice sweetspot for hard disks at the moment is the one you mentioned. That 320GB Seagate SATA2 drive which goes for about $95 including postage from Newegg (or it used to, might be cheaper by now 🙂 . I added three of them. One for the ‘C:” OS boot drive, and another two which I RAID-0’d (and is used for video). I’ve still two other older PATA drives as well left over from before the upgrade (albeit with PATA->SATA converters seeing as how the new MB had only enough PATA interfaces for the optical drives).
I think you’re going in a good direction.