I'm in a bit of a hurry but try my last bit of advice in the Bass Boost EQ etc post.
Hook up an EQ/boost (software) to your soundcard and mix down that way.. however you like it. Everybody's is different anyway and the real deal will come out nice and flat with lots of EQ play for any kind of audio device on all bands. Do a final EQ when it's mixed and learn a little about compression.. careful, it can be dangerous to your mix. Then use a limiter to get your levels as hot as you can without distorting the output. (Check out Steinberg's Mastering Edition and the DSP effects series. They're both absolute whopper packages for not too much money.)
Some people do the final EQ first, some go right into the compression. What you do with compression depends on how good your mixdown turned out. It's supposed to add finishing touches to your sound and make it playable on a variety of audio devices but you can fuck up the whole mix by not being careful.
Don't forget to use real flat-ref monitors even if they're the lowest end you can possibly afford. The bass may come out a little topheavy at the worst but if you set that original card EQ to music you like to listen to it will still come out pretty darned close. Bass guitar usually has some type of compression on it to begin with because the peak dynamics are so hard to handle. Again on the monitors, it's good to have a sub. Tascam has an extremely cheap sub/sattelite combo for about $100 and they're really nothing to sneeze at, especially if you can't afford a pair of Mackies or whatever. Most musicians I've ever know are butt-ass broke at some point (usually more than one) and just want to get their sound out.
Check your panning too beforehand when mixing...a lot of times you can get a good thick sound without overcrowding by simple right or left spacing with delays. Wide delays are also a good substitute for reverb if you notice they're making your sound muddy. If you listen really closely to the radio you'll notice it's basically compression and delay that give it that fat wall of sound sort of warp.
I like to use a little high-freq tingle (spectralizers, aural activators, sonic maximizer etc.) when I EQ the mix but it's artificial axle grease and some people are real turds about that sort of thing. Digital audio is "artificial" anyway so who gives a ffft? So are electric instruments and amplifiers and delays and all that stuff for that matter if you want to get technical about it. Spectralizers give your mix a nice FM stereo sort of gloss.
But you're talking about a simple demo so...try my way and see what you get. I'm really impressed with what such a small amount of money and a bit more time has done for my recordings.