I personally prefer to build computers from components as it allows expension and customization and noise reduction to the hearts content -- but it does require a small time investment, and it does not come with an extended warrenty.
My suggestion would be that you purchase a machine with a mid or "full tower" size case, as it should offer reasonable expansion room for hard drives and PCI cards,and room for air flow. Noise is always a consideration also... at least for us musical types. If you can find a machine with a good, quite case (http://www.quietpc.com/) that would be a big benefit, but there are also aftermarket coolers that can reduce the noise footprint significantly.
But, I got sidetracked on the bread. For the meat, either an i5 or i7 quad core processor would do just swell. For sample libraries, you want lots of RAM of course (I would not go less than 4GB of ram). For the operating system, I suggest Windows 7 64 bit, as it will allow you to utilize over 4GB of ram. If you want things really snappy and quiet, you might look for a system with a solid state drive as the system drive (and then you would put sample libraries and recorded audio on a separate, larger drive). Your PCI support will depend on the motherboard. The issue is that most computers these days with great motherboards are geared toward gamers, and you might be paying extra for a state of the art graphics card that is beyond what you need. Rather than buying an manufacturer made machine (an HP or Dell, ext), you may be better off with a machine put together by a good computer retailer.
I do computer upgrades frequently, and I get parts at great deals from www.ncix.com. They are originally a canadian store I believe, but there is also www.ncixus.com (a way for you to funnel money into your northern neighbors economy). They do custom builds as well. They have this feature called "PC builder" in which you can customize a machine to your specific needs, without much of a "custom build" premium charge at all.
As I'm not currently up to date on motherboards, I can't give specific recomendations on that right now. But if you do decide to use something simular to PC builder, let me know and I can help with part choices.