I agree, when using Creative drivers, if you scale back on what is installed, you can get down to barebone drivers. In fact, just installing the latest drivers from Creative's website is the way to go. That being said, that is still 50 megs X-Fi base install vs. 15 megs for installing all the Xonar software. As for in game overhead, I didn't check memory overhead/usage of my X-fi, but my Xonar didn't use much (8megs). If it is regards to cpu usage, anyone playing with a modern dual core or faster computer doesn't need to worry about it.
Regarding X-Ram, it is indeed used in the old id tech engine games, Doom 3 and Quake 4. No other games other than those and Battlefield 2 take advantage of it. Also, unless I am mistaken, Soundfont support is purely for music creation and midi synth playback, but I'll admit to not knowing much about it.
As for Dolby Headphone, I found when playing Bad Company 2, Crysis, Bioshock 2 and Metro 2033 it gave a more convincing positional audio with my HD555 and PC360 headphones. I found the CMSS-3D to be good, but seemed to blend positional audio together. Some people like the "smoother" audio that CMSS-3D gives, it seemed to blend channels together and flow around the head more. But I personally liked the pinpoint positioning Dolby Headphone gave. Again though, those were on my particular headphones. Other headphones may different, and speakers are something entirely different.
My recommendation is again for a Xonar card. If you end up going that route, look for the Unified drivers if you run across any issues, or want to use drivers specially tweaked for low DPC latency. If you are intent upon an X-Fi card, Auzentech has their stuff together on the driver side more than Creative does. Though modded drivers from Daniel K and others kind of levels the playing field if you want to mess around with hunting down driver packages.