Sound Card Advice Please? Building a new compy

My current computer can't keep up with what I need it to do audio-wise and I've decided to build a new one. This is my first build and though I have plenty of talented friends to help with assembly and picking certain parts, they are all gamer-minded and I need some help choosing components from audio-minded people.

I'm planning on getting an i7 2600 processor/motherboard and can figure out most of the other components around that, but I'm completely lost when it comes to sound cards.

My primary uses in regard to sound are:
1) Sibelius notation software and playback
2) Sonar X1 Producer, with up to full orchestra roster of samples or more (think mock-ups for film scoring) and various effects

It looks like much of what is out there in high-end cards is focused on gaming and involves re-mixing of sound for you, which seems undesireable. Does anyone have any suggestions on what to look for? I have heard that ASIO is important, but I'm not finding many cards that have/use it. 

I'm not too concerned about inputs and outputs beyond plugging in a midi keyboard (I have a usb converter for that now) and a single microphone. Seems like any card should accommodate this. Since I don't anticipate ever mixing anything beyond two channels, I feel like I should avoid anything that can use more - is that smart or does it not even matter?

I've read in some places that an audio interface is preferable, but I don't totally understand how these work except that using one allows you to go beyond the number of inputs/outputs currently allowed by one's computer. I want to keep latency as low as possible and I'm concerned that using a USB interface would not be worth it over using a sound card with ASIO. I am also not averse to the idea of buying an audio interface one or two years down the road if I find myself recording with more inputs.

Any advice is greatly appreciated! Thanks very much!

Thanks, I appreciate the suggestions! I will check those out.