Hello Charles-
I use an RME HDSPe MADI card in my rig, but it seems you are already aware of RME.
The only other MADI card supplier I am aware of at this time is SSL. If you are mainly looking at recording tracks into Reaper without foldback, I would look into the SSL MadiXtreme 64. This card doesn't provide any on board zero latency monitoring or DSP for hardware accelerated effects, but if what you want to do is get audio into and out of your computer for recording live shows it may be just what you want. Plus, it is quite a bit less money than the RME unit or the SSL MX4 with on board DSP. Here is a link:
http://www.solidstatelogic.com/music...madi%20xtreme/
One issue to be aware of with the SSL cards - the MADI I/O is optical only. From the description of the SD9 on DiGiCo's www site, their I/O is BNC only, so you would have to look into optical-BNC conversion options. Both SSL and RME make such a unit, but the additional expense may lead you back to the RME card anyway.
Please don't get me wrong - I love my RME card! The drivers are stable and the card is extremely powerful. From what I've read, the SSL MX4 is great also. But you'd be paying for a bunch of features you don't plan on using.
If you do get the RME card, make sure you get the PCIe card, and not PCI. PCI is a legacy bus now, and some current chipsets (Sandy Bridge for example) don't have native PCI support. They use bridge chips for PCI support, and in some instances this causes popping and crackling problems.
As far as recording computers, everyone will have their own opinion on what hardware to use. As you are planning on building, you must be thinking Windows or Linux. From my experience with the RME card and Windows 7, the drivers for the PCIe version are very stable and work well with no problems. I think it is best to have a separate drive or RAID array that you record to apart from your system drive. Also, make sure your BIOS options are configured properly - turn off any CPU speed limiting features like C1E halt state.
Don't be concerned if you don't know what some or any of this stuff means! None of us did either at first. Jump right on in!
zendrum