Gents,
 
I'm finishing the design on a Sonar DAW rebuild and have a final obstacle: MIDI support.  What I'm looking for is the ability to hear sounds real-time as I trigger them, but am only interested in capturing the associated MIDI data.  What I've done in the past is use an old SBLive card for it's synth engine.  This allows soundfonts to be loaded onto the card, which is ideal when programming/monitoring drums or keyboards.  And the SBLive has a "game controller" input on the card which lets MIDI device connect directly if needed.  Then later on I use the MIDI tracks to trigger high-quality samples.
 
The drawback to this approach is the SBLive itself.  It can cause unpredictable things to happen in a DAW, and is also limited to 48KHz sample rate.   I should also mention I'm using a RME Fireface 400, which apparently has MIDI support, but I've never actually used it since it has no synth engine.
 
So....what would be some alternative methods now days to achieve similar functionality but without an SBLive card?   I already tried loading soundfonts into a plug-in within Sonar, but there's too much latency.  The solution doesn't necessarily have to revolve around soundfonts, but I need a similar kind of workflow that allows me to input and hear sounds "real time" along with a song to create MIDI tracks for processing later on.
 
        Sonic

Why not just record tracks in sonar as synth tracks and plug in whatever vst you want. I don't but a lot of people use TTS to sketch out ideas then replace with other vsts later in the process. Not sure what you mean about FF400 and "synth engine"?? 

Hmm . . . I googled FF400, and found a respiratory face mask, so don't know what audio device that might be ?
It sounds like you just need to get some kind of low latency audio interface to record midi. There are many USB interfaces for as little as $120 (depending of course on how many ins and outs you need to record or monitor at once), they are available with midi in / out patchpoints, complete with midi drivers that work great in Sonar.
 
I haven't used soundfonts for a very long time, used to have a nice collection, but now there are so many VST synths and high end samples available, and the computers of today can handle low latency recording . . . for much longer into the creation process, before you have to eventually adjust the latency to allow for all the mixing stage plugins.
 

The much maligned Soundblaster series did have the ability to run soundfonts from hardware, which is capable of very low latency. That was something that was pretty useful in the days when we used machines with a gig or less of memory and a processor that ran a clock an order of magnitude ormore slower than today's machines. A modern machine should be able to run a rompler from software without excessive latency, and there are a ton of free soundfont players out there. Are you running a seriously antique machine? If not, then maybe you should look into why the latency is so high.

FF400 = Fireface 400 (by RME).  I updated the OP to clarify this.
 
I'll try tinkering with the latency, but past experience leads me to believe I won't be able to achieve a real-time feel.
 
    Sonic

Ahh, so you already have replaced your soundblaster with a top notch RME interface, now we just need to know what your computer specs are to determine why you might have excessive midi latency when trying to create something.
Though, your post subject seems to indicate you are looking for advice on creating a new DAW, or perhaps just getting the most efficient midi usage from what you already have ?

I think what you need to d is learn how to use the hundreds of wonderful VST software synths , nobody has used soundfonts for 12 years or more. we all use the built in or 3rd party VST instruments and you for sure can play them in real time, Many use Sonar and it's VST's live on stage. You should not experience latency using a RME. 

I have both the RME and the SBLive in the system, which is a dual core Pentium 3GHz.

I use TX16Wx recommended by someone here.
available here:
http://www.tx16wx.com/
I cannot let go of some of my soundfonts either; especially the voices in papelmedia.

It's easy. If Sonar see your RME midi ports, then go to add track - instrument track - and choose TTS. Then in input signal choose your RME midi channel ( omni it's good for start ), and you will hear your TTS sound when you play on controller. Rest is for you, you can record, or change sound in TTS.