Fast processor, at least a fair amount of RAM, HUGE hard drive, firewire ports, hi res screen.
I just picked up an ACER Aspire 5004 with a 1.8GHz AMD Turion processor, 1GB RAM, and a 100 GB drive, 1280x800 screen. I got it at Circuit City for under $700. It even has a Dual-Layer multiformat DVD burner. It also supports a second monitor which is very handy for editing. All it's really lacking is firewire ports. I ordered a two-port card from Newegg.com for $3.99 and it should be here in a few days. Vegas runs very spiffy on it, just about as fast as on the 2.66GHz Celeron in my (now dearly departed) desktop PC.