you should use tom's hardware for building your computer, and use another site like the audioholics forum (just an example) for speaker advice. there are many talented people here that will put together an awesome build, and you can take whatever budget you have left and get the speakers you need.
you're going to be looking for powered studio monitors, and just so you know, they get very expensive very quickly. make sure to do lots of research, and find them locally for a listen if you can.
Is this going to be a high-end, professional workstation? Don't spend more than you need to.
From what you've given us, it's hard to determine what exactly you're looking for, so the best we can do will be based on your budget. The best choices for a $1500 non-gaming rig would be an i7-930 processor, X58 mobo with USB 3.0 and SATA III (Gigabyte or ASUS), 12GB triple-channel DDR3 (G Skill, CL 7), a 1TB Spinpoint F3 or 7200.12, 500-700W PSU (I recommend Corsair), and a 5770 (fine for light gaming at 1920x1080).
As far as cases go, Antec and Cooler Master are great choices.
Speakers, monitor, and other parts should be picked based on your own research, I think.
The only audio card I can quickly find with XLR inputs is the M-AUDIO Delta 1010LT. Though it does not have any 1/4 inch inputs it does have MIDI connectivity and RCA based input and output. It also appears to have a removable OAMP if you want to squeeze some more quality out of it. Otherwise soundcards for decent audio inputs are pretty sparse.
For most audio adapters for PCs you will want PCI since most PCIe cards tent to use bridging chips that introduce latency in recording. Also if you want to hear that card really sing you will most likely need move up from generic PC speakers as they will not do that card justice.