Upgrading rear derailer and cassette I recently got what most people would consider a cheap and toy road bike as a gift. It came with very primitive RD and cassette (shimano SIS? I think). I enjoy upgrading my stuff, my question is, would upgrading to a RD such as shimano tourney and another slightly more expensive shimano cassette yield better shifting performance and durability? After adjusting my RD it shifts fairly ok, however I feel that it is very cheap and flimsy.
If its a low quality bike, you're probably best off just selling it and buying a decent bike to begin with -- there are good used road bikes for reasonably low prices.

The first problem is that cheap road bikes likely don't even have cassette+freehub systems -- they have freewheels. High quality freewheels aren't cheap, and you do need a freewheel remover of the appropriate type. It also won't really affect the shifting performance unless your rear cogs are not actually ramped for shifting.

The rear derailleur upgrade may make it stay in adjustment better, but the rear derailleur is really a "yes man" to the shifters - the shifters tell how much cable to pull and the derailleur does that subject to the limit screws not letting you shift into the spokes or out of the rear cluster,the B screw's body angle and the cable tension being good. To get good performance out of this, you'd need good shifters and a good derailleur. 

Unfortunately, shifters are not cheap and have to be matched to the number of speeds in the back (unless you run friction shifters where the shifting is a continuous motion which you trim), and will likely cost more than the bike itself. 

Your best option in terms of performance is to just adjust everything as well as you can and run the bike as is. When things start failing, get a new bike. If the things are so low quality that Tourney is an upgrade, pretty much anything you do will be an improvement in performance and durability but it will also cost more than the bike is worth (and you will likely to be able to buy a used bike with better quality components once you take into account the component costs (possibly even excluding tools+labor costs)).