You might want to check out a Compaq system that Best Buy has for $200 that includes vista basic, along with 512 megs of memory, a 120 gig hardrive, and celeron 3.46 cpu. The motherboard for the system has an Intel 945g chipset which normally supports core2 duo cpus, depending on the bios version. You can probably add an e6600 cpu and more memory. It has a separate pci-e slot. Vista basic normally sells for $85, so for $115 more, you get a case, hardrive, memory, cpu, and dvd rw.
What is your budget? I do essentially the same thing you are and am using AMD X2 3800 and 4200. In fact I put together an inexpensive computer, AM2 3800, 1GB of RAM, and an ASUS with 6150 onboard video strictly for capturing, editing, and burning. Capturing, burning, and rendering take a long time no matter what processor/mobo/RAM you have and it's best not to be doing anything else when doing these things. Dealing with analogue is especially a pain in the a$$ since you have to manually stop the capture. You might want to think about putting together 2 less expensive machines, could do it for about $1000. If you want to do one better machine, AMD would be worth considering if you're on a tight budget.