What I do, is I use this card for my music, games, movies, etc. but for MIDIs, I have an old Pentium III computer with an ISA SoundBlaster AWE64gold sound card. It's a much older card and is not as good with games and media, but it actually has a better synthesizer than the SB X-FI HD.
Yes, all of the AWE32/64 cards *had* near professional quality synthsizers [1], but they are a bit old now. The super awesome synth of the AWE32 was a benefit Creative Labs got when they bought the professional synth company EMU out years ago. If you are a professional and you are willing to use a standalone synthesizer, then you can consider getting a rack module for a few hundred dollars off of ebay:
http://www.vintagesynth.com/emu/emuxl1.php [2]
or you can get the highly recommended EmulatorX3. EmulatorX3 is without a doubt absolutely awseome and it can run on your main computer or a standalone separate computer:
http://www.emu.com/products/product.asp?category=501&subcategory=168&product=17681
Also recommended is the kx project that Poidude mentioned. If your emu10kX based sound card/OS is supported by the kx project, then you can do almost anything with it...
[1] - The Wave Table synthesis of the AWE Cards was near awesome, but the cards were a bit lacking in real time filters that are available in the more recent and expensive EmulatorX and stand along rack products for professionals.
[2] - While these stand alone rack modules work well for pro-audio, they probaby won't work that great for gaming because they don't tend to come with a General MIDI bank. You would have to find such and expansion card and that might be difficult . On the other hand, EmulatorX3 does come with a General MIDI bank if I remember correctly, but it is a software only solution.
Best,
Shawn
Lastly, this whole dicussion is a bit on the silly side since I don't think most modern games actually use MIDI and SoundFonts/Wavetable/SoundBanks anymore. Instead, I think game writters tend to compress their sound tracks into variations of MP3/OGGs files which are then decompressed by by main CPU and send straight to the Digital to Analog chip on the sound card thereby completely voiding the need for Wavetable or MIDI. If I understand this correctly, mostly only professional musicians use MIDI these days.
Note: I am a professional *.NET programmer and hobiest musician. While I don't know much about writing video games, I do know a bit about music and Creative Labs...