Firstly, there is some pretty good advise on this thread.
I played dry for years and just used notes, harmonics etc or what ever to fill up the space, but I have now started to use some effects. I now have a Sansamp bass driver deluxe and a Boss GT10b. As you are on a budget, forget the boss as it is quite pricey & probably not a good place to start as there are almost too many options. But from that I have been able to try MANY different effect as it pretty much has it all......so....
I would suggest some sort of distortion or fuzz/overdrive. I would also suggest a chorus as there are a lot of songs that have a bit of chorus on the bass. Compression is also great for the bass, especially if you are slapping at all, but it won't really fill out the space, just compress your sound. Flanger is also quite cool and can add some cool atmosphere.
But I also agree with the above comment, don't go crazy on them untill you have a real feel for them (and so does the rest of your band). Subtle can often sound better than a full on chorus sound eg. I also agree to really get the most out of them it would be a real good idea to try whatever you buy out before the gig, have a real play & maybe a band practice.
Youtube is great for having a listen to specific models and different types of effects. If you can't decide before the gig what you really want, just play dry!!! Effects can add a nice touch, but not really needed. They are also expensive, so make sure you really know what you want before you spend your hard earned cash.
But, they are fun to try, that is for sure. Either way, good luck on your gig & enjoy whatever you get!
