Doesn't the myspace player compress everything down to 96kbs anyway?
Even if it doesn't, unless you're producing lushly produced orchestral post rock or some shit, a lot of people won't be able to tell or won't care about the difference anyway, I've stolen a lot of mp3s through myspace and they were all 96kbs so just knock it down to that. If it sounds unbearable to an impartial third party coming out of a shitty pair of computer speakers, you are probably making music good enough that you don't need myspace to promote your band anyway.*
Remember, this isn't the medium in which your music is going to be listened to, this is just a taste to get people to buy your album/go to your shows. People aren't going to be too judgemental if the quality is a bit rough anyway, anyone that cares enough would be able to tell it was because of file compression and not just because of shitty mixing and mastering anyway,
*I should probably clarify here that I am assuming your music isn't utterly unlistenable rubbish to begin with.