eSATA is just the external version of SATA. A lot of people use those Western Digital MyBook external enclosures, which have models that include usb 2.0/firewire400, and usb2.0/eSATA. The eSATA models are actually cheaper so if you’re looking for external storage I’d recommend it.
However Mike has the right idea with his setup having a 250gig system drive to run programs from and separate 500gig drives to put media on. Quad processors are great for rendering times, but just last night I was assembling my new home PC with the dual core intel e8400. I haven’t even installed Vegas yet but from the benchmarks I’ve seen it should do pretty well. Also that piano black finish is a nice choice if you’re meeting with clients. Who says only macs can have fancy cases!