It depends on what you are looking for with "good" surround sound.
Room EQ will give you better tonal consistency for all speakers, especially if they are different makes and models. The tonal consistency helps the speakers image, so when sound is split panned between speakers, it sounds like it is coming from positions between the speakers.
Going from 5.1 to 7.1 would give you more enveloping sound.
Bipole or dipole surround speakers would give you a more diffuse sound that doesn't appear to come from the surround speakers. This is most useful if the surround speakers are too close to you, and the ability to hear the position of the speakers is distracting. Dipoles are the most controversial. They give the most diffuse sound, which is especially useful if they are really close. On one hand, this can give a very convincing diffuse effect. On the other hand, it can be unnatural and fatiguing at times. Also, dipoles can have cancellation problems with 7.1 setups, and they lack bass, due to the way they work. With 7.1 setups, many say standard monopole features will do the job. If they are close, bipoles would help, but I would reserve dipoles for close 5.1 surrounds.
Some tiny surround speakers can be really anemic. Nicer ones give a more full range sound. Most AV receivers can reroute the bass from surround speakers to your mains or your sub(s). The nicer ones will let you tune the frequency. Having surround speakers that are the same as your mains is probably overkill, but there is a benefit to moderately sized ones.