It all depends on how many instruments you'll be recording simultaneously and how elaborate you're going to get during production. I've successfully recorded and produced entire songs with ProTools 8 on a Pentium IV 2.8GHz machine. I had to dedicate its entire soul to the cause (following all the optimizations from the ProTools install manual), but I made it work and it worked fine.
Knowing my own short-comings with this setup, though, what I would suggest is much different, though. I'd still make the DAW its own thing. You want it isolated and off a network with no virus scanner running. When you get the DAW running and everything is working great, the only thing you need to worry about ever upgrading is your recording and production software; no OS updates, no driver updates or anything. DAWs should not be multi-purpose machines if you want them to run consistently and predictably. For example, you don't want your acrobat software or virus scanner popping up to let you know there's an update in the middle of a recording session. You don't want a virus scanner slowing down any part of the process or scanning your VIs while they are working to produce sounds for you. This is why it should be an isolated machine.
That being said, if you're a one or two instrument at a time guy (like me), and you're using drums from your VI toolset, I would suggest no less than a dual-core processor at 2.8 GHz or above.
i7s are more than ideal for a DAW.
i7-9xxs are great if you're going to run 64-bit or 32-bit recording/production software because you have 8 threads available for processing effects and virtual instruments. This also helps when processing during your mixdowns. You can pick up an i7-950 at microcenter for $200 these days. You could go with either a mATX or ATX X58 motherboard.
RAM is key and relatively cheap. The amount you need depends on the recording setup. Until recently (July-ish 2010), ProTools wasn't even officially supported in a 64-bit Windows 7 environment. So since 32-bit OSs only "see" a theoretical maximum of 4GB (in reality when you check available memory in Windows 32-bit you'll see from 3GB to 3.5GB max), the recording/production software you run and the supported environment (32 or 64-bit) will determine the amount of RAM you need. Most X58 boards will let you run with 4GB with no problem. If you're running in a 64-bit environment, you could start with 6GB and just add as necessary.
You don't need much video capability. You need an OK $40 or $50 video card.