You can get 303ish sounds with the acid style synths by tweaking decay, pitch and accent in the sequencer.
Long decay sustains the note and it slides when the pitch changes on the next note.
Ofcourse, this is not exactly 303 way but I've done some freaky basslines for fun.
It would be really nice to see Spark vst having multiple midi outs, like audio now has, so you could actually use
one of those sweet 303 vsti clones in your daw and sequence them with spark.
Basically, each mixer channel should have it's own midi-out.
Ofcourse you can now use spark to sequence any vsti but only with one midi out and therefor you cant sequence
synths and drums at the same time.
Unless there is some option I havent figured out.