If you have the IRQ openings, you actually should be able to use both cards together in your computer.
If you don't want to do that, then yes - you can do this without actually using a whole keyboard. You can get an external synth, usually known as a "tone module" or a "sound module". You'll still need a MIDI interface (MPU-401) card, though, if your current soundcard doesn't have one. It sounds like right now you're more interested in listening to MIDIs than playing them - and if that's the case, you'll want to make sure that the sound module is General MIDI compatible. I have
a Roland MT-32 for instance, which is from before Roland adopted the GM standard. So, I can't play standard MIDI files on it (unless I want to remap the instruments).
A good sound module (I'm a fan of Roland, for instance) will still be of use to you, even if you decide you want to start using it for your own music. But it's also not a really cheap solution. Just for fun, I checked Yahoo Shopping. I found three flavors of the Roland Sound Canvas (a very popular module), ranging in price from $349-$589. And I can't even get the link to the $349 model to load.
Next, I checked eBay for some older Sound Canvas models (there are a million of them, and I'm not clear on the differences, other than the SC-55 is the earlist model - I think).
Here's an MGS-64. It's a new store demo. MSRP of $695. The opening value is $199, and so far there are no bids.
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1492372223
An
SD-35. Pretty old. Might be a lot of fun to play with. Current bid is $52.
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1493134176
An SC-88. Looking for an opening bid of $200, or "Buy it now" for $300.
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1492859273
And an SC-55. Current bid is only $76, but there's a reserve and the seller has no feedback rating (and, he's got the sunglasses icon from a recent name change).
http://cgi.ebay.com/aw-cgi/eBayISAPI.dll?ViewItem&item=1492412640
Lastly, you could get a software synthesizer, which would use your soundcard's wave output to simulate a popular sound module such as the Sound Canvas. You can find out about those, plus hear demos of a lot of different synths and softsynths, here:
http://www.queststudios.com/quest/compare2001.html