The wiki has some pages under the category: Hardware. This is a great place to start. When I based my research off of those recommendations I have never had a problem with having a plug and play experience. After you have read over that and learned some of the basics you can then come to these forums and search specific hardware models to see if anyone else has had experience good or bad with it. Up to a point the better/faster your core machine is the better experience you will have with testing out LMCE and hopefully sticking with it and adding devices. Below are some of my experiences with hardware that may give you some starting search points.
I prefer nVidia over ATI or Intel graphics cards. L3mce has written some really nice detection scripts that pick up cards of all makes but I find the nVidia driver packages available to offer more configuration options than ATI which just works.
The magic number for getting smooth UI2 w/ alpha blending in 1920 x 1080 for me has been 256MB, I am sure with current drivers and some tinkering a good 128MB card could run 1080 but it probably won't be capable out of the box.
In all of my tinkering with old junk hardware I have come across a single onboard sound card that was not automagically configured. For a true audiophile I am sure nothing short of optical out would be sufficient but my untrained ear can not tell the difference either way.
Go for gigabit support on your second NIC card, do not skimp on this. The extra couple bucks here makes a giant difference later when you start adding network devices.
Plan ahead for adding IO, if you are comparing two similar main boards and one has twice the PCI slots do it. Though a lot of USB devices are supported having a spare PCI slot to add a soundcard for squeeze or a capture card for DVR/cameras is always nice.
Be very patient when asking for help, if you are willing to be patient, follow directions exactly and contribute with feedback from your experience you will find more than enough support to get you going.
You can get to specific user pages by following my link below, many users have posted detailed information about their systems including specific model numbers.