looking for good, reliable, but not necessarily cutting edge
Your title says "upgrade," but I gather you are NOT planning to upgrade an existing PC, but instead buy a new one. In that case, just search this forum. There are HUNDREDs of posts on this in the past six months. Short version: it doesn't really matter. Just get LOTS of hard disk space, and the fastest CPU your budget can afford. Vegas works with just about anything, and fine-tuning won't buy you all that much extra performance when rendering or previewing to an external monitor. These are the two areas where a faster computer helps. The actual editing can be done on anything. I still regularly capture and edit on a 450 MHz Pentium. Perfectly adequate. I just don't render on it.